Coming into this semi-final, he had already spent nearly 16 hours on court.
Nadal said: "I think it was 2-2 in the first, 15-0, that I felt it".
"That was not a tennis match at the end, no?"
The 32 year-old Spaniard retired on Friday night when tailing the towering Argentinian 6-7, 2-6, with a recurrence of knee trouble that made it impossible to continue.
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The midfielder is in the final year of his contract in Paris and has rejected offers of a new deal from the French giants so far. The 28-year-old England midfielder, who joined from Sunderland in 2011, had agreed a previous five-year deal in April 2015.
"The pain on the knee is always very similar", he said. The two that preceded the Wimbledon classic were easier victories, a semifinal rout en route to the French Open title and a four-set victory in last year's U.S. Open semifinals, when Nadal dominated the last three sets. But at the same time I have to keep going and working hard to get more opportunities.
The Wimbledon champion, who ended a two-year grand slam drought by winning at the All England Club, has now won 13 sets in a row in NY. "You could imagine very hard for me to say good-bye before the match finish".
I didn't take any pleasure in watching Nadal go through the motions in that second set - especially in case he did more damage.
Djokovic said he changed his shirt 10 times in his quarter-final victory over Millman in a match which was held up for more than six minutes at 2-2 in the second set when the Australian left the court to change his clothes.
Reaching a second consecutive Grand Slam final suggests the 13-times major champion is almost back to his best, though he will need to be even more clinical against Del Potro, who has only dropped one set in six matches in NY.
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Not so fast , came the response from Xander Schauffele on Friday as he backed up a first round 63 with a second round 64. The crowd was much thinner for McIlroy, but those fans saw quite a show.
Djokovic missed last year's tournament owing to an elbow injury.
Nadal leads the series 11-5, though del Potro routed him in straight sets in the 2009 semifinals.
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.
"He's a big match player and I'm sure the final will be a thrilling match". "I had my biggest memories on the tennis court playing on this court ... but I was a kid".
Both Nadal and Djokovic are in position for a two-slam season and a chance to inch closer to Federer's men's record of 20 Grand Slam titles.
While Djokovic has been able to routinely beat Nishikori in their previous meetings, Nadal and Del Potro have only met on four occasions since 2013.
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I feel like she has a big chance because the players which stayed [in the tournament], I feel like she can beat all of them". You can't control the weather, can't control what the tournament is going to do. "So obviously I know her game really well".