Novak Djokovic has said that his opposition to vaccines may prevent him from returning to tennis after the coronavirus pandemic.
There have been calls for all tennis players to be vaccinated when the season eventually resumes, although the scientific community has warned a vaccine is at least one year away.
But such a plan would leave World No.1 Djokovic facing a dilemma.
"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," Djokovic said in a live Facebook chat with several fellow Serbian athletes.
"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.
"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."
Former World No.1 Amelie Mauresmo last month said tennis should not resume unless players can be vaccinated. She tweeted: "International circuit players of all nationalities plus management, spectators and people from the four corners of the world who bring these events to life. No vaccine = no tennis."
As with every other professional sport, the tennis world has been thrown into disarray due to the Covid-19 outbreak, governing bodies having suspended all tournaments until July 13 so far.
Wimbledon has been cancelled for the first time since World War II, and the French Open has been put back four months until late September.
Meanwhile, Rafael Nadal has expressed his frustration that tennis players remain unable to practice due to the coronavirus pandemic while other people have been allowed to go back to work.
Nadal treated fans to an Instagram live session yesterday, during which he spoke to both Roger Federer and Andy Murray about their experiences during lockdown.
The men's and women's tours in tennis have been halted due to the virus.
"I am not playing tennis, I do not have a court at home and I miss it a little," Nadal said.
"I am sticking to my physical routines. From the gym of my academy they were able to bring me some machines when lockdown began so I try to work a little in the morning, a little in the afternoon.
"It is very important to have both the head and the body focused and it is what I am trying to do at all times.
"It's true that I don't understand (some aspects of lockdown) very well because we cannot play tennis when many people are going to work and even more so in our sport, where we keep at a large and safe distance and we play on opposite sides of the court.
"But I understand that we are in a very critical situation, that the government is dealing with something that is unprecedented, and I also understand that the last thing they are thinking of is who can train and who cannot train.
"I understand the situation and obviously there are many things that are not logical but you have to accept the rules."
The Spanish government has extended the state of alarm until May 9, with the entire population confined to their homes, except a small number working in specific sectors.
Spain has been one of the world's worst-hit countries by the coronavirus as the number of confirmed deaths passed 20,800 yesterday, according to official figures.