"I hate retiring but to stay one more set out there playing like that is too much for me", said Nadal.
For del Potro, it was an odd way to return to an important summit.
"Of course it's not the best way to win a match", Del Potro said during his on-court interview.
Novak Djokovic of Serbia celebrates victory following his men´s singles semi-final match against Kei Nishikori of Japan on Day Twelve of the 2018 US Open at the USTA Billie Jean King National Tennis Center on September 7, 2018.
The No. 1-seeded Nadal has a history of tendinitis in his knees, and he's often cited that when withdrawing from tournaments.
Both former champions in New York, Djokovic will be looking to deny the Argentine a second title after he went all the way in 2009. "I just have the chance to play the greatest in this sport, and it's unbelievable for me".
"I love to play against Rafa because he's the biggest fighter", del Potro said after the Nadal left the court.
"I'm really happy to be in the semis again".
In his lengthy efforts to fully recover from his wrist injuries, del Potro almost gave up tennis in 2015. Who could ask for anything more? "That was the bad moment for me", he said.
"Lots of people in this room, including myself, never will think [sic] that at the age of 32 I will be here fighting for titles, fighting for the first position of the rankings". "I didn't expect to get in another Grand Slam final".
"He fights every match from the first to the last point". That was the last thing Nadal needed.
That didn't stop him losing the set, though, and he was forced to save five more break points at the beginning of the second in a 14-minute game.
Del Potro led a standing ovation on Arthur Ashe Stadium when Nadal walked gingerly off court.
Then he returned to the baseline to serve again and eventually finished out the set before stopping.
"I waited as much as I can".
"When you win or when you lose like this", Nadal said, "you come back home with the feeling that you did all the things that you can do". He even barely took any time between points while serving - and he's known as one of the slowest guys on tour. It was so hard to keep playing that way. It was so hard for me to keep playing, I was in too much pain.
Nadal already had the admiration of tennis fans for his relentless approach and unyielding competitive spirit.
Nadal, who reached the last four in NY on the back of brilliant shot-making and incredible movement, looked out of sorts early against Del Potro and struggled to cover the court with his usual tenacity. Thousands in the crowd would lend their voices to the song; yet others would try to drown them out with support for Nadal. But Nadal was so distraught after he pulled out of the match that he had difficulty saying he was pleased to see del Potro advance.