World number one Novak Djokovic has once again declined to reveal whether he has been vaccinated against COVID-19, and has said he is unsure if he will defend his title at next year's Australian Open.

The state of Victoria, where the grand slam event takes place in Melbourne, has introduced a vaccine mandate for professional athletes, although authorities have not yet clarified what the requirement will be for those coming from abroad.

"Things beings as they are, I still don't know if I will go to Melbourne," Djokovic told the online edition of Serbian daily Blic.

"I will not reveal my status, whether I have been vaccinated or not. It is a private matter and an inappropriate inquiry.

"People go too far these days in taking the liberty to ask questions and judge a person. Whatever you say, 'yes', 'no', 'maybe', 'I am thinking about it', they will take advantage."

Djokovic has won the season's opening grand slam a record nine times, including the last three editions.

The 34-year-old last competed at the US Open, where his bid to complete the calendar slam — winning all four majors in the same year — was ended by Russian Daniil Medvedev in the final.

Djokovic, who is tied with Roger Federer and Rafael Nadal on a record 20 Grand Slam titles, said he planned to compete in the Paris Masters, the season-ending ATP finals in Turin, and the Davis Cup before the end of 2021.

"I am following the situation regarding the Australian Open and I understand the final decision (on COVID-related restrictions) will be made in two weeks. I believe there will be a lot of restrictions just like this year, but I doubt there will be too many changes.

"My manager, who is in contact with the Australian Tennis Federation, tells me they are trying to improve the conditions for everyone, both for those who have been vaccinated and those who have not."

However, Victorian Premier Daniel Andrews said on Tuesday, unvaccinated tennis players are unlikely to get a visa to travel to Australia for the Australian Open. 

"I don't think an unvaccinated tennis player is going to get a visa to come into this country and if they did get a visa they'd probably have to quarantine for a couple of weeks," Andrews said in a press conference.