The United States Tennis Association and the US Open are "hopeful" world number one Novak Djokovic will be allowed to play this month's Indian Wells and Miami Masters events.
The US requires international visitors to be vaccinated and proof will be required until at least 10 April.
Djokovic missed last year's US Open because of his vaccination status.
Indian Wells and the Miami Open - two of the most prestigious tournaments on the tennis calendar outside the Grand Slams - start on 6 and 20 March respectively.
In a tweet, the US Open said: "Novak Djokovic is one the greatest champions our sport has ever seen.