Melbourne, Jan 19 (AFP) Japanese US Open champion Naomi Osaka and sixth seed Elina Svitolina survived big scares but displayed fighting qualities to pull back from the brink at the Australian Open on Saturday.

On day six at Melbourne Park, fourth seeded Osaka looked down and out against Taiwanese veteran Hsieh Su-wei before battling back to win 7-5, 4-6, 6-1 and set up a last 16 meeting with Latvia's Anastasija Sevastova, who beat China's Wang Qiang in straight sets.

Pliskova struggled with a painful shoulder complaint that required treatment at every change of ends, and medical timeouts, before somehow beating China's Zhang Shuai 4-6, 6-4, 7-5 as the temperature rose in the centre court sunshine.

They played ahead of Serena Williams making another bow on Rod Laver Arena as she gathers steam in her bid to win a record-equalling 24th Grand Slam title.

The American great takes on unseeded Ukrainian teenager Dayana Yastremska.

Misfiring world number one Simona Halep is also in action, testing herself against Williams' sister Venus on Margaret Court Arena with the winner potentially facing Serena for a berth in the quarter-finals.

Osaka's win equals the 21-year-old's best performance at the Australian Open, where she reached the last 16 in 2018 but lost to Halep.

But she looked headed for the exit when she conceded the first set and was down 4-1 in the second before winning five straight games to force a decider.

"I just didn't want to give up," said the Japanese, who made 42 unforced errors and received a code violation after throwing her racquet.

"I really love Grand Slams so I did anything I could do to stay here a bit longer."

Ukrainian Svitolina also staged an epic fightback and shrugged off an injury scare to reach the fourth round.

- 'Die or win' -

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Svitolina, who scored her biggest career win by clinching the WTA Finals in Singapore last October, received treatment on her neck and shoulder twice during the victory.

"It was very hot conditions, I was struggling a little bit. We left everything on court today. We practise every day for these kind of matches," she said, watched by boyfriend Gael Monfils.

"I just told myself 'you're going to die or win'. It was a mental game. I had to stay strong.

"We were fighting until the last point. I was just trying to stay out there as long as possible."

She will meet either Belgian 12th seed Elise Mertens or 17th seeded American Madison Keys in the next round.

Fourteen-time Grand Slam winner and top seed Novak Djokovic is gunning for a seventh Melbourne Park title and faces 25th-seeded Canadian Denis Shapovalov later, one of the NextGen stars who are out to topple the sport's big guns.

"I expect a really interesting encounter," said Djokovic.

"He will not have anything really to lose, so I'm sure he's going to come out really pumped."

Whoever wins will play either 21st seed David Goffin or 15th seed Daniil Medvedev.

Halep, who is returning from a herniated disc, has limped through her first two matches at Melbourne Park, taken to three sets in both, and knows she will need to up her game to make the last 16.

"Tough one, but for sure I'm not going to run that much," she said of facing the elder Williams sister, who is still going strong 21 years after playing for the first time at Melbourne Park.

"Because it's going to finish fast, the points. She's a great player, champion," she added ahead of the night match on Margaret Court Arena.

Others in action Saturday include men's fourth seed Alexander Zverev against young Australian Alex Bolt and Japanese star Kei Nishikori, who faces Portugal's Joao Sousa. (AFP) ATK

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