MADRID -- Defending Olympic tennis champion Rafael Nadal pulled out of the London Games on Thursday with an undisclosed injury.

"I am not in condition to compete in the London Olympics and therefore will not travel as planned with the Spanish delegation to take part in the Games," the third-ranked Spaniard said in a statement.

Nadal's withdrawal will keep an Olympics tennis streak alive, at least on the men's side. No player (male or female) has ever won consecutive Olympic gold medals in singles.

Nadal did not mention any specific injury, 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.

Nadal has not played since losing in the second round of Wimbledon to then 100th-ranked Lukas Rosol, one of the most surprising results in the tournament's history.

At No. 100, Rosol was the lowest-ranked player to beat Nadal in a Grand Slam tournament. It was also the earliest Nadal has exited a Grand Slam tournament since 2005.

He is 42-6 in singles events in 2012. He won the French Open title and was runner-up at the Australian Open.

"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," he said. "I tried to hurry my preparations and training to the very last minute, but it was not to be."

Nadal, who won the singles tournament at the 2008 Beijing Olympics, was set to be the flag bearer for Spain during the opening ceremony.

"(This) is one of the saddest days of my career as one of my biggest ambitions, that of being Spain's flag bearer in the opening ceremony of the Games in London, cannot be," Nadal said. "You can imagine how difficult it was to take this decision."

It wasn't immediately known who would replace Nadal on Spain's tennis team or who would carry the flag at the opening ceremony in his place.

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 in London, however. Federer and Switzerland teammate Stanislas Wawrinka are the men's defending champions, while American sisters Venus and Serena Williams won gold in the women's doubles in Beijing.

Information from ESPN Stats & Information and The Associated Press was used in this report.