Melbourne, Jan 17 (AFP) Top seed Simona Halep was forced to dig deep to stay in the Australian Open on a day of epic battles Thursday that saw Japanese pair Naomi Osaka and Kei Nishikori survive but 2014 champion Stan Wawrinka crash out.

The world number one Romanian staggered through against American Sofia Kenin 6-3 6-7 (5/7) 6-4, but it was far from convincing and her second three-set test in a row.

"I have no idea how I won this tonight, it's so tough to explain what happened on court," said the French Open champion, who now meets Venus Williams after the 38-year-old rolled back the years to beat France's Alize Cornet.

In another titanic battle, Canadian Milos Raonic ousted Wawrinka in four tightly-contested sets that all went to tie-breaks, while Nishikori was stretched to a draining five sets against Ivo Karlovic before sinking to his knees in relief.

Seventh seed Dominic Thiem limped out when he retired while losing 7-5 6-4 2-0 to young Australian wildcard Alexei Popyrin.

Thiem went through a first round five-setter against Benoit Paire that finished in the early hours of Wednesday morning and he never looked 100 per cent.

US Open champion and fourth seed Osaka had it much easier, sweeping past Slovenia's Tamara Zidansek 6-2 6-4 with the roof closed on Margaret Court Arena after rain interrupted the opening major of the year.

The 21-year-old's breakthrough at Flushing Meadow over Serena Williams made her a new standard bearer for tennis in her homeland, Asia and the next generation of women, and she is working hard on living up to the hype.

- Energy-sapping -

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"I thought I served alright, this was my first time playing her and I'm just happy to win to be honest," Osaka said after setting up a clash with Taiwan's Hsieh Su-wei.

In contrast, Asia's top-ranked men's player Nishikori needed an energy-sapping 3hr 48min to get past big-serving Croat Karlovic 6-3 7-6 (8/6) 5-7 5-7 7-6 (10/7) in humid conditions.

"It was a tough match which could have gone both ways. He almost had it for sure," said the eighth seed, a three-time quarter-finalist at Melbourne Park.

On Rod Laver Arena, 16th seed Raonic emerged the victor against Wawrinka, a three-time Grand Slam champion who is on the comeback trail from injury.

The Swiss, unseeded for the first time since his debut in 2006, fell to the Canadian 6-7 (4/7) 7-6 (8/6) 7-6 (13/11) 7-6 (7/5).

Raonic, who got to the Australian Open semi-finals in 2016 before losing to Andy Murray, will next face France's Pierre-Hugues Herbert who beat last year's semi-finalist Hyung Cheon of South Korea.

Despite being seeded six and seven respectively, neither Elina Svitolina or Karolina Pliskova has progressed further than the last eight at the Australian Open.

But self-assured Svitolina is bubbling with confidence after dismantling Viktoria Kuzmova 6-4 6-1, while Pliskova woke up after losing the first set to race past Madison Brengle 4-6 6-1 6-0.

Other women through included Italy's Camila Giorgi and China's impressive 21st seed Wang Qiang. Belgium's 12th seed Elise Mertens lined up a third-round showdown with American Madison Keys.

Men staying in the hunt included 11th seed Borna Coric of Croatia and Spain's 23rd seed Pablo Carreno-Busta.

They were all in action ahead of 23-time major winner Williams who plays Eugenie Bouchard later, while six-time champion Novak Djokovic turns back the clock in a night match with Jo-Wilfried Tsonga on Rod Laver Arena.

Bookies have installed Osaka as second favourite to lift the title after Williams, who in her first tour match since losing the US Open final delivered an ominous warning in a 49-minute opening round thrashing of Tatjana Maria.

Williams is bidding to match Margaret Court's record of 24 Grand Slam crowns on her first return to Melbourne Park since winning in 2017.

Djokovic, gunning for a record seventh title, has a fairytale meeting with Frenchman Tsonga in a rematch of their 2008 final that the Serb won. AFP SSC SSC

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