The Australian Open reached the quarterfinal stage on Tuesday with Rafael Nadal facing the challenge of young giant-killer Frances Tiafoe and Petra Kvitova up against Ashleigh Barty in the late round of matches.

Roger Federer slayer Stefanos Tsitsipas earlier tore through to the semi-finals in a four-set win over Roberto Bautista Agut - but for Kiwi Michael Venus his involvement in Melbourne ended abruptly.

Michael Venus eliminated in quarters

Kiwi Michael Venus and South African doubles partner Raven Klaasen's run at this year's Australian Open has ended.

The sixth-seeded pair were knocked out in the quarterfinals at Melbourne Park on Tuesday night, losing in straight sets (6-4, 7-6) to Leonardo Mayer of Argentina and Portugal's Joao Sousa.

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The last time these teams met, Venus and Klaasen won a five-set thriller at Wimbledon.

Hewitt snubs Kyrgios for Davis Cup

Nick Kyrgios has been left out of Australia's Davis Cup qualification tie in Adelaide next week – just as he predicted.

Lleyton Hewitt has reportedly informed the world No. 52 that he will not be required for the revamped format against Bosnia and Herzegovina.

Under the new Davis Cup format Australia must beat Bosnia and Herzegovina to qualify for the new Davis Cup finals, to be played at the end of the 2019 season in Spain.

The Herald Sun first reported Kyrgios is expected to be overlooked in favour of Alex de Minaur, John Millman, Alex Bolt, Alexei Popyrin and John Peers.

Australia's Nick Kyrgios takes a drink during a break in his practice session at the Australian Open tennis championships in Melbourne, Australia, Sunday, Jan. 13, 2019. (AP Photo/Aaron Favila)
Australia's Nick Kyrgios takes a drink during a break in his practice session at the Australian Open tennis championships in Melbourne, Australia, Sunday, Jan. 13, 2019. (AP Photo/Aaron Favila)

Kyrgios' selection snub comes just days after he denied a rift with Hewitt and the two-time slam champion conceded that Kyrgios had recently failed to meet his new performance and behavioural standards.

Kyrgios said there was "not really" a rift between him and Hewitt and replied "sure" when asked if he supports Hewitt as his skipper.

After losing in the first round of the Australian Open, Kyrgios also admitted he didn't expect to be picked for the tie having slipped behind de Minaur and Millman in the pecking order as he has free-fallen down the ATP Tour rankings.

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"I don't think I'd be picked," Kyrgios said after confirming that he would make himself available to play.

"There's a lot of good players right now. Obviously de Minaur, Millman, they're all playing great. There's a lot of depth, which is pretty good for us.

US star's stunning comeback

Anastasia Pavlyuchenkova sent US star Danielle Collins a strong message to open up their quarter-final clash, overcoming a marathon 11 deuce game before going on to win three break points.

But the American wasn't ready to let her deepest run at a grand slam slip between her fingers that easy.

Danielle Collins reacts against Anastasia Pavlyuchenkova. Photo / Getty
Danielle Collins reacts against Anastasia Pavlyuchenkova. Photo / Getty

Collins rocked back in the second set 7-5 to send the Russian to a deciding third set — and that's when she really kicked into gear. Collins blasted through two breaks before Pavlyuchenkova could blink and had the Russian at 5-0. A fatal error — a long forehand return — was Pavlyuchenkova's demise as she dropped the final set without hardly making a dent.

Collins will face the winner of tonight's clash between Aussie Ash Barty and Petra Kvitova in the women's semi-final.

Stefanos Tsitsipas through to semis

Stefanos Tsitsipas has sensationally downed Roberto Bautista Agut in a fourth set triumph, advancing to his first Open semi-final.

"It feels like a fairytale," he said. "I'm living the dream."

Bautista Agut served up a double fault to hand Tsitsipas a match point opportunity late in the fourth set, but the Spaniard scraped his way through with a thundering forehand winner to send it to a tiebreak.

But from there, the Greek youngster ran away with it, blasting Bautista Agut 7-2 in the break before falling to the deck in awe of his achievement.

At 20 years and 168 days, Tsitsipas is the youngest player to reach an Australian Open semi final since Andy Roddick in 2003. He is also the first Greek player in history to reach a grand slam semi.

Serena coach pulled up

Serena Williams' coach Patrick Mouratoglou has been whacked with a coaching warning during Greek freak Stafanos Tsitsipas' quarter final against marathon man Roberto Bautista Agut.

Mouratoglou has helped Tsitsipas develop through his Mouratoglou tennis academy, but the 20-year-old's father Apostolos remains his official coach.

Patrick Mouratoglou receives a coach violation in the quarterfinal match between Stefanos Tsitsipas of Greece and Roberto Bautista Agut of Spain. Photo / Getty
Patrick Mouratoglou receives a coach violation in the quarterfinal match between Stefanos Tsitsipas of Greece and Roberto Bautista Agut of Spain. Photo / Getty

Mouratoglou was sitting in Tsitsipas' player's box when the No. 14 seed was warned for coaching by chair umpire Marja Cicak.

The moment came as Tsitsipas stole the third set with a late break of serve before members from his player's box shouted out to him appearing to tell him to take a bathroom break in between sets.

"As he left the court, the box of Tsitsipas actually was communicating something down and Tsitsipas actually just got a code violation for coaching in that little break there as well which is interesting," Channel 9 commentator Sam Groth said.

"Not sure what the message is. Telling him to get off the court. It is warming up quite a lot now so the word might have come down to take the chance to get off the court and get cooler."

Mouratoglou had earlier this week reiterated his public calls for coaching to be allowed on both the WTA and ATP Tour.

"It's not allowed but it's tolerated because the coaches are doing it every day," he said on the Eurosport podcast.

"If it's not too obvious the chair umpire lets it happen. And if it's too obvious the chair umpire, in general, says slow down or I will have to give you a warning."

Aussie's 'jaw dropping' brain snap

Aussie juniors talent Rinky Hijikata has sensationally been knocked out of the Australian Open after simply choosing not to play match point.

The No. 5 seed appeared to simply forget the rules for this year's boys singles where let serves that land in are played on as regular points.

In a costly brain snap, Hijikata whacked his opponent Zane Khan's first serve straight into the ground – clearly forgetting that the point was live, despite the ball clipping the net on its way through.

Rinky Hijikata of Australia has been knocked out in dramatic fashion at Melbourne Park. Photo / Getty
Rinky Hijikata of Australia has been knocked out in dramatic fashion at Melbourne Park. Photo / Getty

The incredible moment happened on match point when Khan was serving at 5-4 in the second set.

Hijikata's costly error not to return the serve ended the match abruptly and painfully for the Aussie.

Khan was awarded the win 6-4 6-4 and Hijikata simply had to accept his defeat, despite clearly looking bewildered when the match was awarded to the American.

The Aussie teenager may have lost his second round boys singles match, but he will be back in action in the boy's doubles on Tuesday afternoon.

Hijikata had recently payed in the qualifying draw to try and sneak into the main draw of the Australian Open where the controversial let rule does not exist.

Woodbridge backs Busta after bust-up

It sparked the tantrum of the tournament but the umpiring decision that ruined Pablo Carreno Busta's Australian Open was incorrect, Aussie doubles great Todd Woodbridge says.

The No. 23 seed's furious blow-up — which saw him booed off the court — has completely overshadowed an epic five-hour marathon which saw Japanese superstar Kei Nishikori progress through to the quarterfinals on Monday night.

Pablo Carreno Busta of Spain throws his bag as he shows his frustration after defeat in his fourth round match against Kei Nishikori of Japan. Photo / Getty
Pablo Carreno Busta of Spain throws his bag as he shows his frustration after defeat in his fourth round match against Kei Nishikori of Japan. Photo / Getty

The match was turned on its head by a controversial chair umpire decision following a late linesman's call which ruled Carreno Busta would not have returned a Nishikori backhand.

Carreno Busta furiously argued he had stopped playing because of the overruled line call, but the point was still awarded to Nishikori.

Pablo Carreno Busta of Spain shows his frustration after losing his fourth round match against Kei Nishikori. Photo / Getty
Pablo Carreno Busta of Spain shows his frustration after losing his fourth round match against Kei Nishikori. Photo / Getty

"The call was wrong. It should have been a replay," Woodbridge told Channel 9 on Tuesday. "I know my colleagues Jim Courier and Lleyton Hewitt, last night when they saw it, they thought the umpire made the right call.

"Tom Sweeney — who is a very good umpire — has made an assumption that point is going to be over because we saw Carreno Busta go to his left and leave the open court where the winner was going to.

"However, had he propped and gone the other way, then you have to replay the point. You can't assume that it's going to be a winner ... It's just too close."

Day Nine preview

Day nine of the opening Grand Slam of the year sees Rafael Nadal look to move another step closer to winning an event he has only claimed once before among his 17 Grand Slam titles.

That was in 2009 when he beat Roger Federer in the final.

The Swiss master has already been eliminated this year, and Nadal will have his hands full against American Tiafoe in a night match on Rod Laver Arena.

Tiafoe, 21, has already accounted for fifth seed Kevin Anderson and battled past Grigor Dimitrov to make his first ever Slam last eight.

"He has everything. He's quick. He serves well. Very quick forehand," said second seed Nadal, who has not dropped a set so far. "He's a very dynamic player, aggressive one. Of course, he's dangerous."

The winner will play either the man who shocked Federer in round four, Greek 14th seed Stefanos Tsitsipas, or Spain's Roberto Bautista Agut, who ended the tournament, and possibly the career, of the injured Andy Murray.

Bautista Agut, who has been stretched to five sets in three of matches so far, is wary of the gifted Greek, who is in a Slam quarterfinal for the first time.

"He's one of the top players in the world. He will be one of the best, for sure. He's playing at a very high level," he said.

Greece's Stefanos Tsitsipas. Photo /AP
Greece's Stefanos Tsitsipas. Photo /AP

Czech eighth seed Kvitova goes into her clash with Barty having won all nine of her matches this year, including a three-set win over the Australian in the final of the Sydney International warm-up tournament.

She also has the experience of winning two Wimbledon titles, but will have to deal with a crowd backing only one winner.

"It will be fun one," she said of meeting the 15th seed again. "I mean, Australians, of course, will be on her side, but that was the same in Sydney. So I'm kind of used to it."

The other quarter-final pits the unseeded pairing of Anastasia Pavlyuchenkova and Danielle Collins against one another, with both players seeking Grand Slam breakthroughs.

Russia's Pavlyuchenkova is playing in her second quarter-final at Melbourne Park, but it is the first for Collins who had never won a Grand Slam match before this year but stunned second seed Angelique Kerber in round four.

"Everybody gets their shot at the pie. Right now I'm certainly getting mine," said the American world number 35.