Novak Djokovic has not withdrawn from the ATP Cup despite claims he was almost sure to do so, Tennis Australia said Tuesday, as other top players led by Naomi Osaka began arriving ahead of the Australian Open.

There has been serious doubt as to whether men's world number one Djokovic would travel to Australia, having refused to confirm if he has been vaccinated against the coronavirus.

An unnamed member of his team told Serbian newspaper Blic that they were "99 percent" sure that he would not play in the ATP Cup, which comes ahead of the Australian Open, adding to the intrigue over whether Djokovic would be at the first Grand Slam of the year.

But Tennis Australia told AFP the Serbian had yet to pull out of the teams event, which is due to start on Saturday in Sydney with Djokovic drawn to play Norway's Casper Ruud on the opening day.

"We haven't heard of any withdrawals from Team Serbia for the ATP Cup," a spokesperson said.

Tennis Australia has organised charter flights to bring international stars into Melbourne and Sydney, with the first arriving Tuesday.

Defending women's Australian Open champion Osaka was among those to touch down, but Djokovic was not on board, TA said. Players can also opt to fly commercial.

Nine-time Australian Open champion Djokovic could win a record 21st Grand Slam title if he plays at Melbourne Park, but to enter Australia he and members of his entourage have to be vaccinated.

The 34-year-old has previously expressed his opposition to the vaccine and his father Srdjan said in late November that Djokovic would probably not play the Australian Open, accusing the organisers of "blackmail".

The ATP Cup has already been hit by several players testing positive for Covid, including Russian world number five Andrey Rublev and Canada's Wimbledon semi-finallist Denis Shapovalov.

Spanish superstar Rafael Nadal, who was due to play a warm-up event in Melbourne next week, also contracted the virus. They all played in an Abu Dhabi exhibition tournament this month.

Three female players at the same event also tested positive -- Olympic gold medallist Belinda Bencic, US Open champion Emma Raducanu and Wimbledon quarter-finallist Ons Jabeur.

Tennis Australia has organised the charter flights to help minimise the risk of getting the virus on the trip over, with all passengers having to provide proof of a negative test in the 72 hours before flying.

The Australian Open is due to start on January 17.