A weary and wounded world No.1 Rafael Nadal has retired from his US Open semi-final while trailing Juan Martin del Potro 7-6 (7-3) 6-2, his second retirement at a grand slam this year.

Nadal showed signs of distress almost from the start as he limped noticeably after a gruelling 69-minute opening set.

He called for the trainer during the changeover on serve at 2-1 in the second.

The defending champion had his knee massaged and re-taped and bravely continued through the second set before telling the umpire he was unable to continue.

“It’s not the best way to win a match,” said Del Potro. “I love to play against Rafa because he’s the biggest fighter in this sport.

“I’m sad for him.”

Nadal’s stamina and famous fitness have been put to the test on the sweltering Flushing Meadows hardcourts as he has logged nearly 16 hours through five matches.

Nadal also quit during his quarter-final against Marin Cilic at the Australian Open in January.

Japan’s Kei Nishikori meets Novak Djokovic of Serbia in Friday’s second semi-final with a spot in the final against Del Potro on the line.