Kei Nishikori out of Australian Open

Darren Walton
Kei Nishikori has withdrawn from the Australian Open

Australian Open boss Craig Tiley is confident the season's first grand slam won't be reduced to last star standing with many of the game's biggest names remaining uncertain starters.

Japanese ace Kei Nishikori added to the concerns on Thursday when he withdrew with a wrist injury.

"My rehab is going well but I am just not ready 100 per cent to come back yet in best-of-five set matches," Nishikori said.

The former world No.4 and US Open finalist's absence follows doubts over the fitness of grand slam giants Rafael Nadal (knee), Novak Djokovic (elbow), Andy Murray (hip) and Stan Wawrinka (knee).

Djokovic, Murray and Wawrinka haven't played since Wimbledon almost six months ago, while Nadal withdrew from this week's Brisbane International after canning a Christmas training block in Majorca.

Djokovic and Wawrinka also pulled out of last week's exhibition event in Dubai, but are hopeful of returning to competitive tennis at the Open after joining Nadal in committing to the Tie Break Tens at Melbourne Park next Wednesday.

Djokovic also announced on Thursday that he would take part in the Kooyong Classic exhibition, raising hopes his longstanding elbow problem won't cruel his chances of challenging for a record-setting seventh men's Open crown from January 15.

"I spoke to Novak and he's already got every hour of the day planned between when he arrives and the start of the Open, so he'll be ready," Tiley said.

'And the same thing with Rafa, who's on his way down here now, and he'll be ready.

"Not dissimilar to where we were in 2017. We've got 128 men and 128 women."

In addition to doubts around four of the sport's so-called Big Five and Nishikori's withdrawal, world No.8 Jack Sock quit his Hopman Cup match on Tuesday after hurting his hip in a nasty on-court fall in Perth.

The women's Open could also be hit with superstar withdrawals as new tennis mums Serena Williams and Victoria Azarenka contemplate their next move.

Williams was coy about her plans to defend her title after losing to French Open champion Jelena Ostapenko in Dubai last week.

While pleased with her performance in her first match since having her first child in September, the 23-times grand slam champion said she was unsure if she was ready for the Open.

"I'm in touch every day with her and with her management team. She wants desperately to be here," Tiley said.

"I still believe she will be. She has made it very clear to me that this is her favourite event and that's why she'll do everything she possibly can to be here.

"Her whole life evolves around getting ready for the Australian Open right now. She wants to be here and when Serena wants something she normally gets it.

"So we hope that that is the case."

Two-time Open champion and fellow former world No.1 Azarenka pulled out of the Auckland Classic for "personal reasons" as the Belarusian continues to battle for custody of her son with her former American partner.