The US Open is the only Grand Slam that world No 1 Rafa Nadal has yet to win and the Spaniard admits he has found it tough physically to raise himself at the end of the season.

The Open is held at the end of August, when Nadal, a clay-court specialist, has often been feeling the strain of the season.

"There are a lot matches for me, a lot of tension in those three months, it's a very important part of season for me and I put everything in there. And after that, it's normal (to) go down," he said.

"In 2002 and 2007 I had problems with my knees ... in 2008 I arrived perfect physically but destroyed mentally after winning at Roland Garros, Wimbledon, Toronto and the Olympics. So it was crazy for me, a very hard season.

"And last year was another. I injured my abdominal (muscles) in Montreal, so I played here in Cincinnati with an injured abdominal and with every game, the tear of the abdominal got bigger.

"I finished the US Open destroyed, so for me it was amazing result getting to the semifinal last year," he said.

Nadal is targeting success at this year's event at Flushing Meadows but says he has no clue as to whether his current good form will lead him to finally claim the US Open.

"You can do everything perfect but that is not why you are going to win or going to be playing well in that moment," he said. "Physically I'm perfect. Give me two weeks to break everything."