Novak Djokovic says he’s "preparing" for the U.S. Open later this month as though he’ll be allowed to compete, but the 21-time Grand Slam winner on Thursday withdrew from the Canadian Open over the country’s COVID-19 vaccine mandate for travelers.

Similar to the U.S., Canada does not allow unvaccinated foreign citizens to travel to the country, prompting Djokovic to withdraw from the tournament.

Thursday’s news could be a foreshadowing of what’s to come later this month at the U.S. Open, where Djokovic is hoping to earn a record-tying 22nd Grand Slam title and his fourth on the hard courts at Flushing Meadows in New York.