Serena Williams and Naomi Osaka face their toughest tests so far when the Australian Open enters the last 16 stage on Sunday, while Novak Djokovic faces a battle to recover from a potentially tournament-ending injury.

Djokovic has been given the maximum amount of time to recover from a midriff muscle tear that left him grimacing on virtually every shot during his attritional, five-set win over American Taylor Fritz late on Friday.

The Serb world number one, looking for a record-extending ninth Australian Open crown, will feature in the late-night match on Rod Laver Arena against big-serving Canadian Milos Raonic.

Djokovic, who chose not to practise on Saturday to seek medical advice and treatment, said he couldn't guarantee he'd be able to play on Sunday.

"I'm very proud," Djokovic said after the match, which was halted midway through to eject fans ahead of a state-wide, five-day coronavirus lockdown.

"At the same time, sad and worried, because it's definitely something serious happening with my injury. I don't have much time to recover for the next match."

By contrast, the most flustered Japan's number three seed Osaka looked in her third-round straight-sets victory over Ons Jabeur was when a butterfly landed on her nose.

But the 2019 Melbourne Park champion is likely to face a sterner test in her maiden clash with last year's beaten finalist Garbine Muguruza, the 14th seed, to open proceedings on Rod Laver Arena.

The pair have five Grand Slam title between them but have surprisingly never met on court before.

Williams, chasing a record-equalling 24th Grand Slam crown will follow that pair on to the cavernous, spectator-less main court take on in-form seventh seed Aryna Sabalenka.

Williams has never before played Sabalenka, who has collected more trophies than anyone else since Roland Garros last year but never reached the quarter-finals of a Slam.

"Well, she hits very hard. She has a big, big power game. She's a big girl. Strong like myself," said Williams of Sabalenka.

"So I think it will be a really good match -- with no crowd."

Second seed Simona Halep will be looking for revenge against teenager Iga Swiatek, who beat the Romanian easily on the way to a maiden Grand Slam title at Roland Garros last year.

The Pole dropped just three games in a 69-minute drubbing of Halep in the last 16 in the French Open and the two now meet at the same stage in Melbourne to open the night session on Rod Laver Arena.

Swiatek is now on a streak of 10 straight-sets Grand Slam wins and, at the age of 19, has reached the last 16 of a Slam on four of her eight appearances on the biggest stage.

In the men's draw, US Open champion Dominic Thiem came from two sets down to beat Nick Kyrgios and silence a vociferous John Cain Arena on Friday, and he will find a fan-free Rod Laver Arena a massive contrast in decibel levels when he faces Bulgarian 18th seed Grigor Dimitrov.

Russian qualifier Aslan Karatsev has been the surprise of the tournament so far, not dropping a set on his Grand Slam debut on his way to a fourth-round encounter with Canada's 20th seed Felix Auger-Aliassime on Margaret Court Arena.

The world number 114 blitzed eighth seed Diego Schwartzmann into submission 6-3, 6-3, 6-3 Friday with a barrage of winners.

On nine previous attempts to reach a Grand Slam event, the injury-dogged Russian fell in qualifying but he finally broke through to reach the Australian Open main draw, and now stands one win away from a quarter-final against Thiem or Dimitrov.

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