Rafael Nadal is withdrawing from the U.S. Open because of knee problems, according to a tweet by tennis commentator Patrick McEnroe. Nadal later confirmed the news in a tweet.
The 26-year-old Nadal has battled knee issues recently. He also withdrew from the Olympics, where he had been scheduled to carry Spain's flag in the opening ceremony. He won the gold medal at the 2008 Beijing Games and would have been defending that title.
Nadal won the 2010 U.S. Open to complete his career Grand Slam. He was the runner-up to Novak Djokovic last season.
Without Nadal, there are three clear favorites for the title. Roger Federer, who again became the No. 1 player in the world again after his win at Wimbledon, could add to his resume as the best player of all-time. Djokovic could defend his title. Or Andy Murray, who just won the gold medal at the Olympics, could break through and win his first major. The U.S. Open begins Aug. 27.