Spain's Rafael Nadal said Wednesday that he will not participate in the U.S. Open, the final Grand Slam event of the tennis season, due to a nagging knee injury.

"I am very sad to announce that I am not ready to play the U.S. Open in NY. Thanks to my fans for their support and specially, the New Yorkers," Nadal announced.

"I am sorry since I always found great crowds and great support, but I have to continue with my recuperation and preparation to be ready to play in the right conditions."

This latest withdrawal is Nadal's fourth since his shocking early-round exit from Wimbledon at the hands of then-100th-ranked Czech Lukas Rosol, a loss considered one of the biggest upsets in that prestigious tournament's long history.

Nadal subsequently pulled out of the London Olympics and key U.S. Open tune-ups in Toronto and Cincinnati, each time indicating that he was still recovering from his injury and was not in condition to play.

In different points in his career, Nadal had been forced to pull out of the other three Grand Slam events due to injury, but this is the first time he was eligible to participate in the U.S. Open and decided to withdraw.

Nadal, who won that hard-court major in 2010 and reached the final last year, had hinted in recent days that he was not sure whether he would be physically able to compete at this year's Open.

The year's second hard-court Grand Slam gets underway on Aug. 27 in New York with Serbia's Novak Djokovic as the defending champion and Switzerland's Roger Federer and Scotland's Andy Murray his main challengers.

Nadal's ranking fell to No. 3 in the wake of his five-set loss to Rosol and will likely fall further due to his absence from one of tennis' major tournaments.

In a statement, U.S. Open tournament director David Brewer said "Rafa has informed us that he will not be ready to compete at the U.S. Open this year. We hope to see him back on the court soon and look forward to his return to New York next year."

The Spaniard began the summer on a high note by capturing a record seventh French Open title with a victory over then-world No. 1 Djokovic in the final.

Nadal was forced to withdraw from the 2004 and 2009 Wimbledon tournaments, the 2006 Australian Open and the 2003 and 2004 French Open events with various injuries. EFE