World number three Naomi Osaka joined Serena Williams and Simona Halep with injury niggles ahead of the Australian Open Saturday as defending champion Sofia Kenin blamed her groin strain on spending two weeks in coronavirus quarantine.

The season's first Grand Slam has had an unusual build-up after the pandemic torpedoed much of last year's schedule and gave players only a week to play warm-up tournaments after their mandatory quarantine in Australia.

Japan's Osaka, the US Open champion and 2019 Australian Open winner, withdrew from her Gippsland Trophy semi-final against Elise Mertens, blaming a long-standing shoulder problem.

"Anyone that's followed me for a bit knows that I've had a slight shoulder thing since 2018 in Beijing," she said. "It kind of flared up again because I played a lot of matches back-to-back.

"But, yeah, my main focus is hoping I can rest enough before the Open."

Naomi Osaka pulled out of an Australian Open warm-up tournament with 'niggling injury'
Naomi Osaka pulled out of an Australian Open warm-up tournament with 'niggling injury' AFP / David Gray

Osaka had looked in fine form powering past Irina-Camelia Begu 7-5, 6-1 in 76 minutes on Friday.

She joined Williams as a late withdrawal after the American 23-time Grand Slam winner pulled out of her semi-final against world number one Ashleigh Barty with a shoulder problem Friday.

Williams, 39, who withdrew after beating fellow American Danielle Collins 6-2, 4-6, 10-6 in the Yarra Valley Classic, said Saturday that the injury wasn't serious.

"I feel pretty good. I've gotten a lot of treatment already on my shoulder. But I'm super-confident it's going to be great," she said.

However, Williams added: "It's definitely something that I'm going to have to deal with for the fortnight."

World number two Simona Halep was hampered by injury in her Gippsland Trophy defeat
World number two Simona Halep was hampered by injury in her Gippsland Trophy defeat AFP / David Gray

Spanish world number two Rafael Nadal has also been struggling with a lower-back problem that has kept him out of the ATP Cup.

Two-time Australian Open champion Victoria Azarenka was another casualty, opting not to play her quarter-final clash with Anett Kontaveit at the Grampians Trophy due to a lower-back injury.

World number four Kenin said not being able to practise properly during quarantine had left her "completely sore" after returning to action.

"After two matches, my leg is completely sore," said Kenin. "You can see that being in a room for two weeks, not playing, practising, it's not the same as playing a match clearly.

"People who haven't played matches for two weeks, it's obviously not the same."

Due to Australia's strict rules for incoming travellers, players underwent a compulsory 14-day quarantine on arrival but were granted daily five-hour blocks for training and treatment.

Some players, including Azarenka, had to endure a hard lockdown and were not able to train at all after coronavirus cases were detected on their charter flights.

"Today it's better. I'm not going to complain about that," she said. "I want it to be better for the Australian Open," said Kenin.

World number two Halep was also hampered by a lower-back injury as she was thrashed by Ekaterina Alexandrova 6-2, 6-1 in the Gippsland Trophy quarter-finals.

"I struggled a little bit from the air-conditioning with my low back," she said.

"The muscle got a little bit blocked. But I did some treatment last night, this morning, so I'm getting better now. It's nothing dangerous."