NEW YORK (AFP) - Juan Martin del Potro reached the US Open final on Friday (Sept 7) 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.

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.