Juan Martin del Potro reached the US Open final on Friday after defending champion, and world number one Rafael Nadal quit their last-four clash with a knee injury.

Third seed Del Potro, the champion in 2009, was 7-6 (7/3), 6-2 ahead when Nadal retired.

The Argentine will face either Novak Djokovic, the 2011 and 2015 winner, or Japan's Kei Nishikori in Sunday's final.

"I hate to retire," said Nadal who has endured a series of knee injuries throughout his career.

He also retired trailing in the fifth set of his Australian Open quarter-final against Marin Cilic in January.

"It's not a match when one is playing, and the other is at the side of the court."

Nadal, bidding for a fourth title in New York and 18th major, had spent the best part of 16 hours getting to the semi-finals.

That effort took its toll with the Spaniard twice needing to have his right knee bandaged before calling it a day after the conclusion of the second set.

"Well, of course, it's not the best way to win a match. I love to play against Rafa because he's the biggest fighter in this sport and I don't like to see him suffering," said 29-year-old Del Potro who will be playing in his second Slam final. "I am sad for him, but I'm also happy to do my best."

"It means a lot to be back in the final," added Del Potro after his sixth win over Nadal, all on hard court. "I didn't expect to get to another Grand Slam final in my favorite tournament."

"I had my biggest memories on this court in 2009 when I beat Rafa and Roger (Federer, in the final). I was a kid then; now I am much older."