Novak Djokovic, the 21-time major champion and former world No. 1, has withdrawn from the US Open due to travel restrictions into the United States as a result of being unvaccinated against COVID-19.

Djokovic made the announcement Thursday morning, shortly before the draw was to be revealed, tweeting, "Sadly, I will not be able to travel to NY this time for US Open."

He wished luck to his fellow players and said he would "keep in good shape and positive spirit and wait for an opportunity to compete again.''

Sadly, I will not be able to travel to NY this time for US Open. Thank you #NoleFam for your messages of love and support. ❤️ Good luck to my fellow players! I'll keep in good shape and positive spirit and wait for an opportunity to compete again. ?? See you soon tennis world! ??

— Novak Djokovic (@DjokerNole) August 25, 2022

After winning Wimbledon in July, Djokovic had stated he had hoped the U.S. would change its policies, allowing him to play at the year's final major, and appeared to be training for the tournament. He posted a video to Instagram on July 30 of himself practicing on a hard court.

"I am preparing as if I will be allowed to compete, while I await to hear if there is any room for me to travel to [the] US," he wrote alongside the clip.

But Djokovic was forced to withdraw from last week's Western & Southern Open, as he was still barred from entry, as well as the National Bank Open earlier this month due to Canada's similar restrictions.

On Wednesday, the Centers for Disease Control and Prevention's guidance on international travel was updated to say: "Non-U.S. citizen, non-U.S. immigrants: You must show proof of being fully vaccinated with the primary series of an accepted COVID-19 vaccine before you board your flight to the United States. Only limited exceptions apply."

Said Stacey Allaster, the US Open tournament director, in a statement: "Novak is a great champion and it is very unfortunate that he will be unable to compete at the 2022 US Open, as he is unable to enter the country due to the federal government's vaccination policy for non-U.S. citizens. We look forward to welcoming Novak back at the 2023 US Open."

It has been a challenging year for Djokovic, who won three major titles in 2021. After receiving a medical exemption to play at the Australian Open in January and arriving in the country, he was embroiled in an international legal battle that resulted in him having his visa revoked and being deported from the country before the tournament got underway. He was unable to defend his title and later made it clear he was willing to skip tournaments that required him to be vaccinated in order to play or enter the respective country.

"Yes, that is the price that I'm willing to pay," Djokovic said in a February interview with the BBC.

Djokovic played in just one event, in Dubai, United Arab Emirates, before the clay portion of the season started in April. He was replaced atop the rankings by Daniil Medvedev for the first time in February. Djokovic reclaimed the No. 1 spot the following month but lost it again in June.

He won his first title of the year in Rome in May but was unable to defend his French Open title, losing to eventual champion Rafael Nadal in the quarterfinals. Djokovic earned his fourth straight trophy at the All England Club last month but received no ranking points for his efforts as a consequence of the ban on Russian and Belarusian players. He fell to No. 7 in the rankings after the tournament and is currently No. 6.

Having reached the final at the 2021 US Open, Djokovic will lose an additional 1,200 points and his ranking will drop further at the conclusion of the event.

Djokovic is next expected to play for the Serbian team at the Davis Cup or for the European team at the Laver Cup. Ranking points are not awarded at either event. There are no tournaments listed on his current schedule on his website.

A lucky loser from qualifying will replace Djokovic in the draw.

Among the other players who will not be at the US Open for various reasons are No. 2-ranked Alexander Zverev, the 2020 runner-up in New York; 2016 champion Angelique Kerber; 2019 French Open finalist Marketa Vondrousova; Gael Monfils; and Reilly Opelka.

Play is scheduled to begin at Flushing Meadows on Monday.

The Associated Press contributed to this report.