Nadal has a history of knee problems, with tendonitis ruling him out for much of the 2009 season and more recently forcing him to withdraw from a charity match against Novak Djokovic on 14 July.
"Doctors have been checking my knee, and I suffer a tendonitis which will have me away from any practice or matches for 15 days, which makes it impossible to play with Novak Djokovic as we'd expected and announced," he said at the time.
"I was with my doctors on Monday and started already my treatment with the goal to be ready for the London Olympic Games and represent Spain once again."
Five days later he was forced to pull out of the event at Wimbledon, citing the fact he was not in the right "condition".
"I do not find myself in a condition to compete," he said. "It is one of the saddest moments of my career."
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