But questions have also been raised over Djokovic's participation in the French Open, the next Grand Slam tournament on this year's calendar.

France's parliament has just given its final approval to a law requiring people over the age of 16 to have a certificate of vaccination to enter public places, including sports venues.

France's sports ministry said on Monday it would not grant exemptions to its latest rules on vaccine passes, which it noted apply to professional players as well as spectators.

But a spokesperson also said the situation might change before the event is held in May.