Top seed Simona Halep has narrowly avoided being dumped out of the Australian Open in the second round.

Halep came from 4-2 down in the third set against American Sofia Kenin, reeling off four straight games to take a 6-4 win and advance to the third round.

There, she will play Venus Williams, who progressed after a three-set victory against Alice Cornet.

"I have no idea how I won tonight," said Halep.

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"It's so hard to explain what happened on court. She played amazing. I got a little bit injured in the 2nd set but I just fought because I wanted to win and play again in front of you," she told the crowd.

Halep, the world number one and 2018 finalist, has been handed a horror Australian Open draw, with both Kenin and Kaia Kanepi being tough early round opponents, before having to face Williams this weekend. If she manages to get through that, it's likely that Serena Williams awaits in the fourth round.

Serena is in action tonight against Eugenie Bouchard. Bouchard, who won the ASB Classic doubles title with Kenin earlier this month, is trying to rediscover the form which saw her reach number five in the world in 2014.

7.25pm: Milos wins a marathon as nervous girlfriend watches on

Milos Raonic won the longest match of the Australian Open so far as he took four hours and three minutes to overcome Stan Wawrinka in four sets — all of which went to tiebreaks.

It was a gutsy come-from-behind 6-7 7-6 (8-6) 7-6 (13-11) 7-6 (7-5) win for the Canadian, who toppled Nick Kyrgios on Tuesday night.

Wawrinka put up a brilliant fight but fell short in a contender for match of the tournament so far.

Raonic's girlfriend was watching on in the stands and was a picture of concentration as TV cameras found her from time to time. She looked more nervous than Raonic as she rode every point, before bursting into a roar of approval when he won the third set.

"Goodness me, that is Milos Raonic's girlfriend," Channel 9 presenter Tony Jones said as she appeared on camera looking particularly intense.

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"It is a nervous, nervous time not only for the player, but all of those as part of their team."

Raonic's girlfriend is reportedly Belgian model Camille Ringoir.

7.10pm: Wawrinka gets weird, Barty party continues, Osaka blasts through?

Channel 9 commentator Jim Courier couldn't believe his eyes when Stan Wawrinka started returning from a different position on the court so soon after breaking Milos Raonic in the fourth set.

Courier questioned what Wawrinka was thinking when he set up differently on the baseline ready to receive next time around.

"Now I am really confused that Wawrinka is not adopting the successful pattern for his return position," Courier said. "I am flummoxed. This is not next level stuff, this is forgetfulness."

Meanwhile, Ash Barty's impressive Australian Open continued when she won through to the second round of the doubles.

Partnered with Victoria Azarenka, the two defeated the Chinese pairing of Shuai Peng and Zhaoxuan Yang 7-5 4-6 7-5.

Defending US Open champion Naomi Osaka made light work of Tamara Zidansek, winning their second round clash 6-2 6-4.

5.10pm: 'Total shock' in Open madness

Ivo Karlovic miraculously took his match against Kei Nishikori into a decider after losing the first two sets but suffered a heartbreaking blow when he was toppled in a fifth set tiebreak.

The 39-year-old — the oldest man in the draw — produced a vintage serving display and won the fourth set with four consecutive aces to take his total up to 47 for the match.

The game before he iced the set, Karlovic broke No. 8 seed Nishikori to love for the second time as the tennis world reacted with shock to the incredible comeback.

Tennis coach and ESPN analyst Brad Gilbert was among those to express their admiration for Karlovic, tweeting: "Are you kidding?"

Nishikori then staged a remarkable recovery of his down. Serving at 4-4 in the fifth set, the Japanese star was down 0-40 but clawed his way back to win the game before Karlovic held serve to make it 5-5.

Both players held their next service games and the deciding set went to a tiebreak.

Drama struck when Karlovic was down 5-6 in the first-to-10 breaker. A Karlovic ball was called long and Nishikori hit the half volley into the net.

However, the Croatian challenged the call and Hawkeye showed the ball clipped the baseline and was actually in. Instead of replaying the point, it was awarded to Karlovic because the umpire deemed Nishikori would never have kept the ball in play even if he knew it was good.

Nishikori wagged his finger and had a crack at the umpire, but the umpire stood his ground.

That made it 6-6 and then 6-7 when Karlovic won the next point but incredibly, Nishikori won two points on Karlovic's serve and gave himself two match points when he went up 9-7. He thumped a return winner at his first opportunity to clinch a thrilling win.

2.45pm: Svitolina's love match revealed

Ukranian Elina Svitolina took just 67 minutes to thunder past Viktoria Kuzmova and reach the third round.

But after the match, it was the man sitting in her players box that had tongues wagging of a potential romantic pairing.

French star Gael Monfils, who was knocked out yesterday in a five-setter by American Taylor Fritz, was lending his support to Svitolina.

Asked about Monfils sitting in her players box, Svitolina was put in an awkward spot. "Yeah...," she laughed followed by a long pause.

"Everyone saw him so he's supporting me, and hopefully we'll see him more in the future, not only in my box but on the court as well."

Monfils has previously dated fellow tennis pro Dominika Cibulkova and Australian Chelsea Altman, who is now married to AFL player Tyson Goldsack.

On the court, the sixth seed didn't have it all her own way in the first before demolishing Kuzmova in the second set.

Svitolina, a quarter finalist in last years' open, will play Karolina Pliskova after she recorded a 4-6 6-1 6-0 win over Madison Brengle.

Pliskova broke the American player's serve six times in seven chances after dropping the opening set. She also sent down seven aces and had 14 service winners.

The seventh-seeded Pliskova has now won all three times she has played Brengle. Pliskova has advanced to the quarterfinals each of the last two years at Melbourne Park, including a loss to eventual finalist Simona Halep last year.

2.10pm: Seeds crusie through

Women's sixth seed Elina Svitolina has burshed past Viktoria Kuzmova in the second round, with the Ukranian toppling her Slovakian counterpart 6-4 6-1.

She's joined in the second round by No.7 seed Karolina Pliskova, who overcame being down a set early to prevail against American Madison Brengle 4-6 6-1 6-0.

Meanwhile, 21st seed Qiang Wang has made light work of Serbian Aleksandra Krunic, winning their second-round match up 6-2 6-3. Wang broke serve three times to none in the match.

11am: Veteran US scribe challenges Hewitt

Lleyton Hewitt. Photo / Photosport
Lleyton Hewitt. Photo / Photosport

The view from outside Australia of the bitter feud tearing apart our men's ranks is Lleyton Hewitt "has some serious issues" to answer for.

Sports Illustrated's Jon Wertheim believes Bernard Tomic's explosive press conference where he attacked the Australian Davis Cup captain shouldn't be brushed over just because it was Tomic speaking.

"Bernie Tomic is the loosest of canons and might have the least moral authority of anyone outside of the top 100," Wertheim wrote, in his mailbag.

"But here's the thing: his points — about Hewitt's conflict of interest; about Hewitt's almost pathological aversion to walking away from competition; about Hewitt's alienating players — have been raised by others.

"This is the opposite of 'damning with faint praise.' It's praising with faint damnation. If Bernard Tomic is the voice of dissent, maybe you're not so bad after all. But here, Hewitt has some serious issues in laundering. Just because it was Tomic who raised them doesn't negate the underlying points."

Wertheim isn't the only one who believes Hewitt has something to answer for.

The Herald Sun's Scott Gullan wrote the Davis Cup captain should have been on the front foot after Tomic's bombshell claims but instead "hid" in an "underwhelming" exclusive Channel 9 interview where he brushed off the accusations.

10.30 Day 4 preview - Night session is a corker

Seven-time champion Serena Williams is back in action tonight against Canadian Eugenie Bouchard, with Bouchard looking to continue on from here solid form at the ASB Classic in Auckland.

The two have met twice before, with Williams winning both matches. The American is looking to match Margaret Court's record of 24 grand slam titles this season, with the Australian Open being her first opportunity to do so.

And while 2014 Wimbledon finalist Bouchard had fallen down the rankings, Williams told AFP she would not be underestimating the Canadian.

"It will be a great match. She plays really well. She does everything well, and I really like that she doesn't quit.

"You know, people write her off, and she doesn't let that bother her. She continues to fight and do what she needs to do. I think that's really good."

In the men's draw, Novak Djokovic faces Frenchman Jo-Wilfred Tsonga in a rematch of the 2008 final, which Djokovic won.

The world No.1 is going for a record seventh title in Melbourne.