Reuters reports that a weary and wounded world number one Rafael Nadal retired from his U.S. Open semi-final while trailing Juan Martin del Potro 7-6(3) 6-2 on Friday, sending the Argentine back to the Flushing Meadows final for the first time since his 2009 triumph.

Nadal, who had never before retired from a U.S. Open match, showed signs of distress almost from the start as he limped noticeably after a grueling 69-minute opening set.

Grimacing with every sudden move, the 17-time Grand Slam winner 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.

“I think it was two-all in the first, 15-love, something like this in the first set, that I felt (something),” said Nadal. “I said to my box immediately that I felt something on the knee.

“After that, I was just trying to see if in some moment the thing can improve during the match.

“But no, was not the day.

“I waited as much as I can. You could imagine very difficult for me to say goodbye before the match finish but at some point you have to take a decision.”