The world's number one-ranked tennis player squared off against the best player in the world as Simona Halep and Serena Williams met in a blockbuster round of 16 clash at the Australian Open.

Fellow No. 1 Novak Djokovic was also on show against fast-rising Daniil Medvedev, while young guns Naomi Osaka and Alexander Zverev grabbed the headlines for very different reasons.

Schedule

ROD LAVER ARENA

(4) Naomi Osaka (JAP) defeated (13) Anastasija Sevastova (LAT) 4-6 6-3 6-4

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(16) Milos Raonic (CAN) defeated (4) Alexander Zverev (GER) 6-1 6-1 7-6

(16) Serena Williams (USA) defeated (1) Simona Halep (ROU) 6-1 4-6 6-4

(1) Novak Djokovic (SER) vs (15) Daniil Medvedev (RUS)

MARGARET COURT ARENA

(6) Elina Svitolina (UKR) defeated (17) Madison Keys (USA) 6-2 1-6 6-1

(7) Karolina Pliskova (CZE) defeated (18) Garbine Muguruza (ESP) 6-3 6-1

(8) Kei Nishikori (JAP) vs (23) Pablo Carreno Busta (ESP)

MELBOURNE ARENA

(28) Lucas Pouille (FRA) defeated (11) Borna Coric (CRO) 6-7 6-4 7-5 7-6

Serena wins Open thriller

How the tables can turn.

Serena Williams returned from a mid-match slump to sink World No.1 Simona Halep in three sets on Rod Laver Arena.

Halep bounced back from a 1-6 loss in the first set to take control of the match and send the contest to a thrilling decider.

Williams, who dominated the match early on with her thunderous serve and powerful returns, began falling victim to Halep's superior court coverage as the 28-year-old found her groove.

Tennis journalist ben Rothenberg described Serena's performance late in the match as "sluggish" as Halep pestered Williams' service game to a fifth deuce at 3-2.

Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty
Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty

But the 23-time grand slam champ refused to budge and broke back to 4-3, overcoming a ridiculous game of Halep winners to force the Romanian into an unforced error to the net to claim the break.

She continued to power on and left Halep chasing her tail as the seven-time Melbourne Park queen sent speedy returns to the corners.

After a brief scare, Williams found herself in the box seat with a service game to close out the match.

Earlier a wide-eyed Williams had to turn back and return to the tunnel after missing her walk-out cue before the match.

Serena Williams had to turn around after missing her walk-out cue. Photos / Twitter
Serena Williams had to turn around after missing her walk-out cue. Photos / Twitter

Williams walked out as the court announcer introduced the "World No. 1" and sheepishly returned to the door before receiving an ovation from the Melbourne crowd.

It had been 2,164 days since the forever-dominant Williams last faced a world number one.

Zverev responds to racquet abuse

Alex Zverev was blasted out of the Open by Milos Raonic in a match that saw the 21-year-old demolish a racquet in exhilarating scenes.

The German World No. 4, who was ousted in straight sets 6-1 6-1 7-6, said smashing his gear "made him feel better".

Germany's Alexander Zverev smashes his racket in frustration during his fourth-round match against Canada's Milos Raonic at the Australian Open. Photo / Getty
Germany's Alexander Zverev smashes his racket in frustration during his fourth-round match against Canada's Milos Raonic at the Australian Open. Photo / Getty

"I was very angry, so I let my anger out."

The dejected star was particularly blunt when a reporter questioned if racquet abuse was something he did on a regular basis.

"Have you never watched my matches?" he jabbed.

"You should watch my matches."

Zverev gone in three

A fuming Alex Zverev has departed the Australian Open after a fiery display on Rod Laver Arena saw the 21-year-old smash racquets in frustration as he was downed in straight sets by Milos Raonic.

A fightback from the German in the third set forced Raonic to a tiebreak, but the bounce-back was simply too little, too late as Zverev crumbled and fell victim to one of the Canadian's brutal drop shots.

The German star never recovered from his bizarrely sluggish start to the match and despite a late third set rally was eventually dispatched 6-1 6-1 7-6.

Zverev only regained his control in the third set after Raonic had ripped through the first two sets 6-1 6-1 and broken Zverev's serve six times.

The 21-year-old finally began to land some first serves and he forced Raonic all the way to a tiebreaker in the third. The big-serving Canadian eventually triumphed 7-5 in the breaker to set up a quarter final against either Borna Coric or Lucas Pouille.

Zverev loses his cool

World No. 4 Alex Zverev completely lost the plot in an extraordinary outburst following his early brain snaps against Milos Raonic.

The rising superstar dropped the first set to Raonic after a weird start to the match saw each player's serve broken within the first 10 minutes.

A particularly silky Raonic pulled away and capitalised on a number of errors from the 21-year-old and stole the set 6-1. Cleary filthy at his series of errors and failures to find his way into the match, Zverev finally snapped when he went down 5-1 in the second set.

The German star smashed his racquet into the ground nine times during a change of ends, an outburst that's already being labelled one of the ugliest in recent years.

"That's crazy. That's like twilight zone stuff," Tennis legend John McEnroe said.

The American champion said Zverev's stroppy demeanor on court would have been a sports psychologists dream.

"Right now he is not exuding positive energy. He's moping a little bit," McEnroe told Channel 9.

Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty
Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty

"And he's down a set, down a break. Things are not going well. He is also giving positive feedback to a player who doesn't need any more than the scoreboard is giving him. It's a different energy than a lot of other top players would give off. Certainly Rafa wouldn't be doing anything other than coming out of his skin trying to get back into it.

"Just generally speaking the body language, he's got the racquet sort of drooping towards the ground. He's not sticking his chest out at all. He gives himself a sarcastic thumbs up there. A sports psychologist would do a lot with this little clip of Zverev as far as things not to do in this environment."

Fellow Channel 9 commentator Todd Woodbridge said of Zverev's poor performance: "It's all just a little lazy".

'I'm normal too': Osaka's golden interview

US Open champion Naomi Osaka has given another hilarious, squirming post match interview where she made it very clear her grand slam breakthrough has not changed her.

Speaking after her impressive fightback win over Anastasija Sevastova 4-6 6-3 6-4, Osaka said she can still walk around Melbourne without being recognised by her many fans around Melbourne Park.

When further questioned by Channel 9 commentator Sam Smith about her seemingly low profile for one of the biggest stars in tennis, Osaka cringed in the spotlight.

"Where do you like to go? Does anybody recognise you," Smith asked.

She responded: 'No. I'm not like that. I don't think they care".

Smith then couldn't believe the Melbourne public has been missing its chance to grab a photo with the Japanese star.

"No one comes up for a selfie," she asked again.

Osaka responded: "No. Am I going to the wrong place. I don't know."

Smith said: "Are you wearing a disguise?"

Osaka couldn't find the right words to defend her apparent low key Australian Open.

"No. I just walk around and everyone's like normal. I'm normal too."

In the same interview she also said she felt inspired by Stefanos Tsitsipas' win over Roger Federer to continue the next generation of tennis star's hot streak against the old guard of established players.

'Insufferable' Open star 'falls apart'

Naomi Osaka is through to the quarter-finals of the Australian Open for the first time after she had to come from a set down against Anastasija Sevastova.

Both players showed moments of tightening up in a tense final set, but it was Sevasunderdoglet her mental demons overcome her.

The underdog, who is notorious for her stroppy, self-defeated body language, was back to her old tricks after she fell behind in the final set, triggering concern for her emotional state.

Some tennis commentators even said the 28-year-old was "falling apart" on Rod Laver Arena.

In the end she was unable to recover and fell 4-6 6-3 6-4 after taking the first set.

Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty
Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty

Sevastava found herself down 3-1 in the decisive set before she secure the break back in a marathon sixth service game where the scores were levelled 3-3 when the Latvian converted her sixth break point opportunity.

Sevastova cut a forlorn figure just minutes later when her next service game was broken and the No. 13 seed continued to show her immense frustrations at her own unforced errors at a crucial point in the match.

It was all Osaka needed to go on and serve pout the match un just under two hours.

Federer rejects 'changing of the guard' comment

If John McEnroe thinks he can usher Roger Federer out the door that easily he has another thing coming.

The tennis legend's declaration we were "watching the changing of the guard" as Greek youngster Stefanos Tsitsipas eliminated Federer from the Australian Open on Sunday night was met with a stern backhand from the Swiss champ.

"Yeah, sure," Federer said at his press conference, when asked about McEnroe's assertion during an on-court interview with Tsitsipas.

"He's in front of the mic a lot. He's always going to say stuff. I love John (but) I've heard that story the last 10 years. From that standpoint, nothing new there."

While not ready to agree his time as a grand slam contender is up, Federer was happy to welcome Tsitsipas to the "next level".

"I think he's definitely done a really nice job the last year and a half," Federer said.

"I mean before that, too, obviously. But beating Novak (Djokovic) in Toronto, the likes of (Kevin) Anderson and (Sascha) Zverev, now me here. That's what you need to do to get to the next level. He's doing that. It's really nice for him.

"I see him definitely being high up in the game for a long time. That was a good night for him tonight."

Tsitsipas' 6-7 (11-13) 7-6 (7-3) 7-5 7-6 (6-5) victory on Rod Laver Arena sees him progress to a quarterfinal against Spain's Roberto Bautista Agut.

He was also reluctant to read too much into his victory.

"I try to take every match differently. For sure it's a good win against Roger. I mean, we all know who Roger Federer is, what he has done in tennis. But I still have to keep my focus, keep my concentration on further goals that I want to achieve. That's a very good beginning. I need to stay humble," Tsitsipas said.

"This win is a good milestone, let's say good first step, as I said, to something bigger. I do feel like my game is pretty good at the moment. I feel confident. That's very important. I'm really pumped and excited to be competing in the quarterfinals two days from now. I'm really waiting for that moment."

Federer's fourth-round loss follows a similarly shocking round of 16 defeat at last year's US Open to Australia's John Millman.

The last time he went back-to-back majors without making a quarterfinal was way back in 2013, when he lost in the second round of Wimbledon to Sergiy Stakhovsky and the fourth round of the US Open to Tommy Robredo.

Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty
Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty

In a somewhat surprising announcement, Federer plans to respond to his Melbourne Park failure by playing clay court tournaments for the first time in the past few years.

Despite appearing to benefit by a reduced schedule in recent years, Federer appears ready to return to Roland Garros, where he's been missing since the 2015 French Open.

"I'm at a moment where I think it would be nice to do it,'' said Federer, speaking in French. "I can say I have missed it, I did the right thing skipping it last year, the year before as well, and the year before that I was injured. So I felt I wanted to do it again."

Just one of his 20 Grand Slam titles has been on clay — in 2009 when he beat Robin Soderling in the final. He lost the three previous French Open finals to Rafael Nadal.

Federer said he wants to avoid having a long break in the middle of the year before the grass season.

"I feel that it is not really necessary,'' he said. ''So that's how this was decided.''

His fans will hope it's an attempt to help rebuild a world ranking that will fall from No 3 to at least No 6 following this tournament — rather than a sign 2019 is looming as a farewell tour.

Tsitsipas' gamesmanship causes tension

Federer's match against the 20-year-old wasn't without controversy as he bristled against Tsitsipas's call for a trainer during the fourth set.

Federer complained his opponent had been able to call for the trainer 10 minutes before he eventually received his treatment, suggesting the Greek star did not actually have an issue that required medical aid.

Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty
Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty

TV commentators also said it was clear Tsitsipas was simply in need of the trainer because of his lack of conditioning for marathon tennis matches.

Federer complained to the chair umpire and was clearly seething. "Federer not exactly happy at that changeover, having a chat to the umpire about the physio," McEnroe said in commentary for Channel 9.

Tsitsipas added further insult to Federer by taking a long time to walk back out onto court while Federer stood on the service line ahead of his service game.

Tsitsipas had already been warned twice for time violations and a third violation would have seen him lose the first point of that service game.

"He would want to be careful. Won't want to push it too much. The umpire, I think, was lenient there," Aussie legend Todd Woodbridge said in commentary. "It was just a normal changeover and he took a lot longer."

Some tennis commentators said Federer was absolutely right to complain, saying it is unfair for players to receive treatment for cramps or muscle fatigue.

The Australian Open crowd didn't like Tsitsipas' gamesmanship either and he was booed as he returned to the court.

Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty
Stefanos Tsitsipas plays a forehand in his fourth-round match against Roger Federer. Photo / Getty

Federer refused to blame the early umpire controversies for his defeat, saying instead his inability to get a look in on Tsitsipas' serve was the reason he fell short.

Federer went 0-12 on break-point opportunities throughout the match and also failed to break Tsitsipas' serve when they played at the Hopman Cup in Perth earlier this week.

"It's human and it's part of the game," Federer said of the umpire controversies.

"I'm not going to start complaining about linesmen or umpires."

He said his struggles with returning Tsitsipas' serve were "very frustrating" and he confessed to having "massive regrets" about the way he played the match.

Three legends named for Hall of Fame

Li Na is the first player from Asia elected to the International Tennis Hall of Fame.

She joins Mary Pierce and Yevgeny Kafelnikov in the Class of 2019, which was announced Monday at the Australian Open.

All three players won a pair of Grand Slam singles titles — one at Melbourne Park and the other at Roland Garros.

Li retired in 2014 at age 32 because of recurring knee injuries. She won the 2011 French Open and 2014 Australian Open, making her the first tennis player born in Asia to collect a major singles title and helping grow the sport in her native China.

Kafelnikov won the 1996 French Open and 1999 Australian Open, while Pierce was the champion at the 1995 Australian Open and 2000 French Open.

The induction ceremony is July 20 at the hall in Newport, Rhode Island.