Defending gold medalist Rafael Nadal has voluntarily withdrawn from the London Olympics citing a lack of fitness. “Today is one of the saddest days of my career,” the 26-year-old, who was set to carry the Spanish flag during the opening ceremony, said in a statement. No specific injury was mentioned, but Nadal hasn’t played since losing in the second round of Wimbledon in late June, and he canceled a charity match on July 4 because of tendon problems in his left knee. The Olympic tennis competition begins on July 28 at the All England Lawn Tennis Club. Thirty-year-old Roger Federer, who just won his seventh Wimbledon title there on July 8, will return to the grass to try for Olympic gold.