Nine-time Australian Open champion Novak Djokovic moved closer to missing the Australian Open in January after withdrawing from the ATP Cup, which begins in Sydney on Saturday.

He is yet to announce if he will attend the first Grand Slam of the season, after organizers said competitors must be fully vaccinated against the coronavirus.

The Serbian world No. 1 has not revealed his vaccine status and there had been speculation in the media that the 34-year-old was seeking a medical exemption to compete.

Now his decision to skip the ATP Cup, announced by the governing body on Wednesday, has sent speculation into overdrive given the team event is essentially a warmup to the Australian Open.

Many top players missing

Austria's Dominic Thiem, runner-up in 2020, has also decided to skip the event in Melbourne starting on January 17.

He has been working his way back to full fitness after a serious elbow injury, and will instead start his season at the end of January in Argentina.

"I will not play this year at the Australian Open in Melbourne, a city that I love and where I have great memories of unforgettable matches in front of amazing crowds," Thiem wrote on Twitter on Tuesday.

"I will miss the Australian fans but I will be back in 2023."

The 2020 US Open champion is one of a number of high-profile names to miss the first Grand Slam of the year.

Roger Federer is out after knee surgery, and Serena Williams also absent through injury along with a raft of other stars.

Rafael Nadal is a slight doubt after being among a number of top players in the men's and women's game to recently test positive for COVID-19. However, organizers have said they expect the Spaniard to play.

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