Rafael Nadal, the defending US Open champion and world No. 1, retired from his US Open semifinal match against No. 3 seed Juan Martin del Potro.
Nadal was trailing del Potro 7-6, 6-2 when he retired. Earlier in the second set, he told the chair umpire he was going to have to retire due to being "in so much pain."
Del Potro said after the match it was tough to see Nadal "suffering" on the court, but was grateful to advance to his second US Open final.
It's the first retirement in a men's US Open semifinal in the Open Era.