Rafa Nadal says Novak Djokovic will need to abide by ATP Tour rules, should they decide to vaccinate players.
Professional tennis across the globe is suspended until July 13 as a result of the COVID-19 pandemic.
Wimbledon has been cancelled for the first time since World War II, and the French Open has been postponed until late September.
A lack of play has only served to increase the hows and the whys surrounding a return, and how tennis may operate in the wake of the pandemic.
Last month Djokovic outed himself as an anti-vaxxer when asked about a potential return to action.
"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", the world no.1 told a Facebook Live chat.
"But if it becomes compulsory, what will happen? I will have to make a decision".
This hasn't sat well with world no.2 Nadal.
In an interview with La Voz de Galicia, the Spaniard said, "I do not know. I am nobody. No one can demand of anyone. Each one is free. But the truth is if you belong to a circuit, perhaps one has to be governed by the rules that the circuit requires."
Nadal added, "So if the tour obliges to vaccinate to be able to travel in order to protect everyone, Djokovic will have to be vaccinated if he wants to continue playing tennis at the highest level.
"I mean him, but everyone, me too. Each one will have to comply exactly like now we have to stay home.
"If the ATP or the ITF forces us to get a vaccine to play tennis, then we will have to get it. Just as we have restrictions on not taking too many medications for obvious doping control issues.
"It is a matter of following the rules".
Regarding the general public then resuming travel, Nadal added, "You have to be responsible and think about what really matters."