MADRID -- Defending champion Rafael Nadal pulled out of the London Olympics Thursday with an undisclosed injury, leaving the tennis competition without one of its leading stars and robbing Spain's team of its flag bearer for the opening ceremony.

Nadal won the French Open for a record seventh time this year but has struggled with left knee problems at times during the season. Shortly after winning his 11th Grand Slam title at Roland Garros, he lost to 100th-ranked Lukas Rosol in the second round at Wimbledon. Nadal has not played since.

The third-ranked Nadal said Thursday he wasn't in condition to compete at the Olympic tennis tournament -- which also will be held at Wimbledon.

"This is one of the saddest moments of my career," Nadal said in a statement. "I have to think about my companions, I can't be selfish and I have to think of what's best for Spanish sport, especially tennis and Spanish players, and give fellow sportsmen with better preparation the chance to compete. I tried to hurry my preparations and training to the very last minute, but it was not to be."

The 26-year-old Spaniard has won two Wimbledon titles and claimed gold at the 2008 Beijing Olympics.

Nadal did not mention a specific injury in his statement but he canceled a charity match in Madrid on July 4 because of tendon problems in his left knee. He has had recurring knee problems in the past.

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Nadal's absence leaves top-ranked Roger Federer of Switzerland and No. 2 Novak Djokovic of Serbia as the favorites, although Wimbledon runner-up Andy Murray of Britain will again likely have a wave of home support as he tries to reach the final at the All England Club for the second time in less than a month.

Murray lost to Federer in the Wimbledon final on July 8. The tennis gold medal match at the Olympics will take place on the same Centre Court on Aug. 5.

Both the men's and women's doubles champions are set to defend their Olympic titles: Federer and Stanislas Wawrinka for the men and American sisters Venus and Serena Williams for the women.