Whenever the ATP Tour season reopens after the coronavirus-inspired global lockdowns end, it might have to do so without its top ranked player.

Widely regarded to be one of the sport's all-time greats, 17-time Grand Slam champion Novak Djokovic is worried that if the ATP makes compulsory vaccinations mandatory as a condition of participating in a resumed tennis season, he will be faced with a difficult decision about whether he will be able to participate.

According to ESPN, Djokovic said in a Facebook live event on Sunday that, "Personally, I am opposed to vaccination, and I wouldn't want to be forced by someone to take a vaccine in order to be able to travel. ... But if it becomes compulsory, what will happen? I will have to make a decision. I have my own thoughts about the matter, and whether those thoughts will change at some point, I don't know."

As of right now, all ATP Tour events have been suspended through at least July 13. Since tennis is one of the most travel-intensive and international sports in the world, some tennis stars have called for the season to be suspended for even longer than that.

Djokovic did not elaborate on the reasons that he opposes vaccines. He did, however, leave open the possibility that he might join resumed play, noting that, "Hypothetically, if the season was to resume in July, August or September, though unlikely, I understand that a vaccine will become a requirement straight after we are out of strict quarantine, and there is no vaccine yet."

Djokovic, who hails from Serbia, won the last tennis major to be played, the 2020 Australian Open. His 17 major titles place him third on the all-time list, behind only Rafael Nadal and Roger Federer. At the age of 32, Djokovic is still a dominant figure in the sport, and many expect him to end his career with the most Grand Slam titles of any player in history. Every single Grand Slam title in tennis since the start of 2017 has been won by either Federer, Nadal, or Djokovic.

It is widely expected that an effective vaccine against the coronavirus would not be available for months, which places the remainder of the seasons for international sports in particular in serious jeopardy.

Update: The tour, organized by the governing body of men's professional tennis, is called the ATP Tour.