Argentine third seed Juan Martin del Potro will compete in his first Grand Slam final since 2009 after defending champion Rafael Nadal retired from their US Open semi-final.

Nadal was trailing del Potro 7-6, 6-2 after 2 hours and 1 minute of playing time when he retired.

The Spaniard, who suffered problems with his right knee during his third-round win over Russian Karen Khachanov, twice needed medical attention before conceding.

“It was difficult for me to keep playing. I was in too much pain and it was not a tennis match,” said Nadal.

“It was one player playing and one on the other side of the court.

“I hate retiring but to stay one more set out there playing like that is too much for me.”

Del Potro, who led a standing ovation on Arthur Ashe Stadium when Nadal, who looked emotional as he limped between points, walked gingerly off court, will now face face two-time champion Novak Djokovic who coasted to a 6-3, 6-4, 6-2 win over Japanese 21st seed Kei Nishikori in the second semifinal.