Juan Martin del Potro will face Novak Djokovic in the 2018 US Open final, after both won their semis on Saturday (NZ time).
Del Potro advanced, when opponent Rafael Nadal retired with a right knee injury after the second set.
The world number one trailled 7-6 (7-3) 6-2, when he decided to retire - the first retirement in a men's US Open semifinal in the modern era.
"It was difficult for me to keep playing," said Nadal. "I was in too much pain and it was not a tennis match.
"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."
Nadal's stamina and famous fitness had been tested on the sweltering Flushing Meadows hardcourts, where he had logged nearly 16 hours through five matches.
Nadal also quit during his quarter-final against Marin Cilic at the Australian Open in January.
Later, in New York, two-time champion Djokovic beat Japan’s Kei Nishikori in straight sets in the second semi-final. The Serbian won 6-3 6-4 6-2 in 2h 22m.
This will be Djokovic's 23rd Grand Slam final and eighth at Flushing Meadows.
If he beats Del Potro, only Roger Federer (20) and Nadal (17) stand above him on the all-time list for most Grand Slam titles.
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