Washington, Aug 16 : Tennis ace Rafael Nadal's withdrawal from the U. S. Open has raised concerns about his long-term health and fitness.

"It''s a very sad moment for him for sure," USA Today quoted 2009 U. S. Open winner Juan Martin del Potro of Argentina, as saying.

While most players expressed concern about his well being and expect him to be back at full force, others felt his prospects of catching Swiss ace Roger Federer''s record of 17 majors had dimmed because of what appears to be chronic tendinitis in his knees.

"For a guy like Rafa, who puts so much strain on his legs on hardcourts, if he''s going to have these recurring knee problems, it''s going to be less likely for him to catch Roger," Mike Bryan, fresh off his Olympic gold medal with twin brother Bob, said.

Nadal, 26, currently owns 11 title majors, including an unprecedented seven French Open crowns, the report said.

"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 told followers on Twitter.

However, ESPN analyst Darren Cahill expressed confidence in the Spaniard and believes he would bounce back strong.

"When he comes back, I''m pretty sure you''ll see the same Rafa, if not better," he said. (ANI)