A tired and wounded world number one, Rafael Nadal, retired from his US Open semi-final while trailing Juan Martin del Potro 7-6 (7-3) 6-2, sending the Argentine back to the Flushing Meadows final for the first time since his 2009 triumph.
Nadal showed signs of distress nearly from the start as he limped noticeably after a gruelling 69-minute opening set.
Nadal, a 17-time Grand Slam champion, mentioned retirement to the chair umpire midway through the second set, then threw in the towel after dropping the set.
Del Potro said afterwards: 'It's the not best way to win a match, I love to play against him, he is a such a fighter and I don't like to see him suffering. I think was 2-All in the first, 15-Love, something like this in the first set, that I felt it.
When Rafael Nadal finally finished a almost five-hour climb into the U.S. Open semifinals, he thought backward as much as ahead.
The 32-year-old, who also retired from the quarter-final in Australia this year with a muscle strain in his right thigh, called the latest setback more aggressive than ones in the past in that it came on suddenly rather than progressively.
Rafael Nadal retired from his U.S. Open semifinal with right knee pain that has dogged him on and off for years, sending Juan Martin del Potro into Sunday's final against Novak Djokovic.
"Negative thing is two of them have been in quarter-finals and semi-finals of a Grand Slam, close to fight for titles". And while he still had the devastating forehand that's been one of tennis' signature shots and carried him to the 2009 title, repeated wrist injuries had made him one dimensional on the backhand side.
"I hate to retire", said Nadal who has endured a series of knee injuries throughout his career.
"All my career everybody say that because of my style, I will have a short career", said Nadal.
Nadal took a medical timeout for treatment after falling down a break, 1-2 in the second set, and was not moving at 100% thereafter.
Sixth seed Djokovic, a two-time champion at Flushing Meadows, will compete in his eighth US Open final - equalling the record of Sampras and Ivan Lendl.
Del Potro beat Nadal in the last four at Flushing Meadows nine years ago on his way to the title, but had suffered misery in recent contests with the Spaniard, losing three successive slam meetings starting with a defeat at this stage of the 2017 US Open.
"I didn't expect to get to another Grand Slam final in my favourite tournament".
Nadal was able to feel at least some pleasure for Del Potro after all his struggles: 'He's a player that went through lot of issues during his career, like me too.
Nadal's challenge looked doomed even as early as the first set, feeling the injury after just four games, he admitted. After that, I was just trying to see if in some moment the thing can improve during the match. "I was a kid then and now I'm much older".