WATCH: Djokovic withdrew from the BNP Paribas Open following his US Open defeat in September.

Novak Djokovic has resumed training following his US Open defeat to Daniil Medvedev—a loss that ended his quest for a Calendar Year Grand Slam. After withdrawing from the BNP Paribas Open in Indian Wells, he announced plans to take part in the Paris Masters, ATP Finals in Turin, and Davis Cup to end his 2021 season.

Whether he intends to travel to Melbourne for the 2022 Australian Open, remains up in the air for the world No. 1, who expressed concern about potential quarantine restrictions regardless of vaccine status in an exclusive interview with Blic, a Serbian publication.

Australia has taken a consistently proactive stance in suppressing transmission of COVID-19 since the start of the global pandemic, and relegated 72 players into strict 14-day quarantine ahead of the 2021 Australian Open earlier this season.

Martin Pakula, the sports minister for the state of Victoria, has strongly encouraged players to receive a COVID-19 vaccine to optimize their chance of playing the first major tournament of 2022 with minimal restrictions.

Victoria and its Melbourne capital currently has a vaccine mandate in place for all Australian athletes, but has not clarified whether overseas athletes like Djokovic will need to present proof of vaccination in order to enter the country.

For his part, the 34-year-old Serb, who has been openly skeptical of the COVID-19 vaccine and has advocated against vaccine mandates, would not share his vaccine status with Blic interviewer Jelena Medic.

The 20-time major champion is in a three-way tie with Roger Federer and Rafael Nadal after winning the first three Grand Slam tournaments of 2021, and as a nine-time Australian Open champion would be the odds-on favorite to win his 21st major Down Under should he choose to attend.