Rafael Nadal will be sidelined for at least two months due to a partially torn patella tendon in his left knee.

Nadal is missing the ongoing U.S. Open, where he was last year's runner-up, and was forced to miss the Olympic tennis event at the London Games because of the knee. He will also miss Spain's Davis Cup semifinal against the visiting United States next week.

"I have to recover and I'll be back once the pain is gone," said the 11-time Grand Slam champion and former world No. 1 star.

Nadal's been sidelined since losing to unheralded Czech Lukas Rosol in the second round at Wimbledon in June.

The reigning French Open champion Nadal has been plagued by knee problems over the last few years and was forced to skip Wimbledon twice in the last four years.