"It is 99 percent sure that Novak won't go to the ATP Cup. He is training here (in Belgrade) but he has decided to give that tournament a miss," an unnamed member of his team said.

The ATP Cup in Sydney is a team tournament that traditionally kicks off the men's season.

World number one Djokovic could win a record 21st Grand Slam title at the Australian Open, which starts on January 17, but to enter Australia he and members of his entourage will have to be vaccinated and the 34-year-old has previously expressed his opposition to the vaccine.

His father Srdjan said in late November that Djokovic would probably not play the Australian Open and accused the organisers of "blackmail".