Former world number one Novak Djokovic said on Thursday that he will not be able to travel to New York to compete at the U.S. Open, the last Grand Slam of the year that begins next week, having refused to be vaccinated against COVID-19.

Current rules require travellers to show proof of full vaccination to board flights to and enter the United States.

“Sadly, I will not be able to travel to NY this time for US Open,” Djokovic wrote on Twitter. “Good luck to my fellow players! I’ll keep in good shape and positive spirit and wait for an opportunity to compete again.”

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The Serbian previously said he was prepared to miss Grand Slam tournaments that require participating players to be vaccinated against COVID-19.

Djokovic was on the entry list for the Aug. 29-Sept. 11 tournament in New York.

The 35-year-old was also unable to defend his Australian Open crown earlier this year after being deported from the country over his vaccination status.