NEW YORK – Rafael Nadal admitted Sunday that poor scheduling after a knee injury may have set him on the road to his worst-ever defeat in a Grand Slam tournament.

The Spanish powerhouse was hammered 6-2, 6-2, 6-2 by Juan Martin del Potro of Argentina in a semi-final match, wrecking his bid to win in New York and become just the seventh man to lift all four Grand Slam titles.

Del Potro played "unbelievable" tennis and fully deserved his win, Nadal said while admitting that he was way below his best and was not in prime physical condition.

"I came back perfect with the knees, but maybe, because I was few times outside of competition and I started back playing two Masters Series at the top level - maybe was a little bit too much to come back," Nadal said.

"It's much better if you can start a little bit slowly with a little bit small tournaments, not with the Masters Series.

"But I don't have any option, because the calendar says that, and I did."

Nadal's injury problems this year started during the arduous claycourt season when he put excessive strain on his suspect knee tendons while winning tournaments in Monte Carlo, Barcelona and Rome.

He took that with him into the French Open only to lose for the first time at Roland Garros in a fourth-round shocker to Sweden's Robin Soderling.

After that, he took two and a half months out, missing the defence of his Wimbledon crown and only returned to action at the Montreal Masters in early August.

But that was where he started to feel some pain in his stomach muscles, an injury that worsened in the Cincinnati Masters the following week.

Still, he said he never considered not playing the US Open.

"It was tough to play a few matches like this," Nadal said.

"But at the same time, it was important for me to play this tournament, try my best. It's one of the most important tournaments of the year, and I had to try, no?"

Try he did, but Nadal never looked like finding the answers he needed to counter the powerful hitting of the six-foot-six-inch (2.03m) Del Potro, who reached his first Grand Slam final to underline his growing status among the sports elite.

By the end, even his defensive play, the hallmark of his game, looked inadequate, with Del Potro yanking him at will left and right across the court.

Turning to the rest of the year, Nadal said he was hopeful he would be able to quickly get back to full fitness from what he called a "not very important injury."

"I have to see the doctor in Spain," he said.

"I have to do another test, because I didn't want to do a test during the tournament because anyway I'm going to play.

"So I don't know how is the injury right now, but I expect not a lot."

One consolation Nadal can take away from New York is that no matter what happens he is assured of regaining the world No.2 ranking he lost to Britain's Andy Murray last month.