Juan Martin del Potro returned to the US Open final for the first time since winning the title nine years ago after an injured Rafael Nadal was forced to retire midway through their semi-final.
Nadal, who spent nearly 16 hours on court and went through a gruelling late-night battle with Austrian youngster Dominic Thiem in punishing humidity to reach the last four, called it a night after falling two sets to love down, embracing his good friend Del Potro.
Del Potro, who was leading 7-6 (7-3) 6-2, reached his first Grand Slam final since winning the tournament in 2009.
"It's not the best way to win a match," said Del Potro. "I love to play against Rafa because he's the biggest fighter in this sport. "I'm sad for him.
"It means a lot to be back in the final. I didn't expect to get to another Grand Slam final in my favourite tournament.
"I had my biggest memories on this court in 2009 when I beat Rafa and Roger (Federer, in the final). I was a kid then; now I am much older."
He will now face former world No 1 Novak Djokovic, the 2011 and 2015 winner, in Sunday's showpiece.
The players enjoyed more comfortable temperatures of 24 degrees Celsius in New York as both men entered a cauldron-like atmosphere on Arthur Ashe Stadium.
And after an exchange of breaks in the first two games, the opener became perfectly poised until Del Potro unleashed his formidable forehand to move ahead in the ninth game, but he then failed to take two set points when serving for the set.
It went to a tie-break where the standard of play and drama exceeded all expectations, but it was the Argentine who controlled proceedings before seizing his opportunity at the third time of asking to take it after 69 minutes.
The Spaniard, who beat Del Potro this year in the French Open semi-finals and Wimbledon quarter-finals, has looked nothing short of brilliant en route to the last four, but he was forced to take a medical timeout after the third game of the next set to have his right knee strapped up.
Nadal, bidding for a fourth title in New York and 18th Grand Slam, was clearly affected by his troublesome knee which impaired his movement around the court. And Del Potro took full advantage by winning the second set before the defending champion Nadal shook his head one last time and congratulated the giant Argentine.
A downcast Nadal rued his luck as the knee problems that have dogged him through his career re-emerged.
He said: "I think it was 2-2 in the first, 15-0, that I felt it. I said to my box immediately that I felt something on the knee. After that, I was just trying to see if in some moment the thing can improve during the match. But no, was not the day.
"I waited as much as I can. But at some point you have to take a decision. It was so difficult for me to keep playing at the same time that way, having too much pain.
"That was not a tennis match at the end. It was just one player playing, the other one staying on the other side of the court. I hate to retire, but to stay one more set out there playing like this will be too much for me."
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