Rafael Nadal's Grand Slam count will stay stuck at 11 for now, sidetracked by another knee problem. The third-ranked Spaniard withdrew from the US Open yesterday. Tendinitis has kept him out of action since his stunning loss at Wimbledon in late June.
    
The injury already forced him out of the London Olympics, where he was supposed to defend his title and carry Spain's flag in the opening ceremony. "I am very sad to announce that I am not ready to play the US Open in NY. Thanks to my fans for their support and specially, the new yorkers," Nadal wrote on his Twitter account.
    
Nadal is still only 26, but the withdrawals raise questions about the future of a player who has had recurring knee problems in the past.
His 11 Grand Slam titles include a record seven on the red clay of the French Open, yet his hard-charging, hard-hitting style of play takes a toll on his body, particularly his knees.
    
Roger Federer, in contrast, has played in every Grand Slam tournament since the start of 2000, a streak of 51 in a row. In 2009, Nadal missed Wimbledon because of aching knees shortly after falling in the round of 16 at the French Open -- the only time in eight appearances he hasn't won at Roland Garros. He was just the second men's champion in 35 years to decline to defend his title at the All England Club.
    
But Nadal eventually came back stronger than ever from that layoff.  After failing to reach the final at the 2009 U.S. Open and 2010 Australian Open, he won the French to start a run of three straight major titles, capped by completing the career Grand Slam at Flushing Meadows.
Nadal's absence immediately leaves a trio of heavy favorites at the last Grand Slam event of the year: defending champion Novak Djokovic; five-time U.S. Open winner and currently top-ranked Federer; and 2008 U.S. Open runner-up Andy Murray, who won the gold medal in singles at the London
Games by beating Federer in the final.
    
Nadal lost in the U.S. Open final to Djokovic last year, part of a stretch of three straight defeats to the Serb in championship matches at major tournaments. But he seemed to be closing the gap, and at Roland Garros in June, he beat Djokovic in the final for his record seventh
title there.
    
Then came the stunning loss at Wimbledon, and Nadal hasn't played since. On June 28, 100th-ranked Lukas Rosol upset him in the second round at the All England Club. Shortly after that defeat, Nadal canceled a scheduled charity match against Djokovic in Spain, citing tendon problems in his left knee.
    
The two-week U.S. Open starts Aug. 27. "Rafa has informed us that he will not be ready to compete at the U.S. Open this year and has withdrawn from the tournament," tournament director David Brewer said in a statement on Wednesday. "We hope to see him back on the court soon and look forward to his return to New York next year."