- Last year's runner-up Rafael Nadal withdraws from the U.S. Open
- Spanish player has already missed Olympics and North American events
- Nadal lost to Novak Djokovic in last year's final in New York
- Former world number one has a history of knee problems
World No. 3 Rafael Nadal has been forced to withdraw from the U.S. Open due to an ongoing knee complaint, the Spanish tennis star announced on Wednesday.
The French Open champion has a history of knee problems, and has not played since a surprise second-round defeat at Wimbledon in June.
The former world number one was unable to defend his Olympic title in London, and the the 26-year-old had to pull out of last week's tournament in Toronto as well as this week's event in Cincinnati.
"I am very sad to announce that I am still not ready to play and have to withdraw from this year's U.S. Open in NYC," Nadal told the U.S. Open website ahead of the season's final grand slam, which runs from August 27-September 9.
"I am sorry since I always found great crowds and great support, but I have to continue with my recuperation and preparation to be ready to play in the right conditions."
Nadal, who was beaten in last year's final by Novak Djokovic, missed large spells of the 2007 season due to knee problems and had to withdraw from the 2008 end-of-season championship plus the Davis Cup final due to similar troubles.
In 2009, Nadal was unable to defend his Wimbledon title after he was diagnosed with tendinitis in both knees, and he has occasionally missed tournaments since to try to manage the problem.
In March this year, he was forced to withdraw before his semifinal at the Miami Masters tournament due to the injury.
However, he managed to play a full clay-court season, winning three titles before clinching a record seventh French Open at Roland Garros.
Nadal then lost in the quarterfinals of the Halle grass event in Germany, and was beaten at Wimbledon by 100th-ranked Czech Lukas Rosol.