Novak Djokovic could be denied the chance to win a record 21st career Grand Slam at January's Australian Open.
The state of Victoria has imposed a vaccine mandate which requires "authorised workers" to be fully vaccinated against COVID-19 by the end of November.
Djokovic has been militantly opposed to vaccines becoming mandatory for players.
Speaking earlier this year, the Serb said, "I don't think it'll come to that. I hope not, because I've always believed in freedom of choice."
Djokovic added, "And I will keep the decision as to whether I'm going to get vaccinated or not to myself.
"It's an intimate decision, and I don't want to go into this game of pro and against vaccines, which the media is unfortunately creating these days.
"I don't want to be labeled as someone who is against or who is for vaccines. I'm not going to answer the question ... and hope that everyone will respect that."
In April 2020, he had the more staunch view that, "Personally I'm opposed to vaccination, and I wouldn't want to be forced by someone to take a vaccine in order to be able to travel."
According to Melbourne publication, The Age, Tennis Australia chief Craig Tiley had fought hard against a vaccine mandate being imposed upon Australian Open competitors.
“Tiley said we won’t get star players and the state government effectively said ‘suck it up’," a source revealed.
Djokovic's ninth Australian Open title was won in January, with subsequent wins at Roland-Garros and Wimbledon bringing his career Grand Slam total to 20.
A straight-sets defeat to Daniil Medvedev cost the 34-year old a calendar Grand Slam, meaning he must wait until Melbourne - at least - to surpass Rafa Nadal and Roger Federer on the all-time winners list.
It could well be the case that Djokovic has actually been vaccinated.
Pictures emerged over the weekend of him attending a SummerStage concert in New York's Central Park, which required all attendees to be vaccinated.