Defending Olympic champion Rafael Nadal of Spain has pulled out of the London Summer Olympics 2012, slated to begin on the 27th of this month, with an undisclosed fitness problem.The third-ranked Spaniard said in a statement that he has had recurring knee problems in the past. He has not played since he was eliminated in the second round of Wimbledon to the then 100th-ranked Lukas Rosol, one of the most surprising results in the tournament's history. Nadal, who won the singles tournament at the 2008 Beijing Games, was the flag bearer for Spain during the opening ceremony of the London Games.