"But at some point you have to take a decision", Nadal told reporters.
In the sixth game of the second set, he argued with umpire James Keothavong over a line call, telling the British official: "It's all right, i'm going to retire, but you were not right on this one".
For del Potro, it was an odd way to return to an important summit. While an nearly five hour quarter-final victory over Dominic Thiem, it would be fair to state concerns over the winning player in their subsequent match, but the quirks of the US Open schedule mean that the Spaniard will have had nearly 72 hours to recover from these exertions - virtually unheard of recovery time in the latter stages of professional tennis tournaments.
Del Potro led a standing ovation on Arthur Ashe Stadium when Nadal walked gingerly off court. He'd defeated Nadal in the semis that year, after all, before snapping five-time champion Roger Federer's 40-match tournament win streak in a five-set final. "Like thousands of other tennis players around the world I'm trying to be the best I can be".
Djokovic reached the final at Flushing Meadows after breezing past Kei Nishikori in the last four, beating the Japanese star 6-3, 6-4, 6-2. Wimbledon champion Djokovic, playing in his 11th US Open semi-final, claimed a 15th win in 17 clashes against Nishikori.
The slower courts, oppressive weather and a long succession of arduous matches took a heavy toll on Nadal, who also pulled out of his Australian Open quarter final in January when he was behind to Marin Cilic. "And it was so hard for me to keep playing at the same time that way - you know, having too much pain, and that was the tennis match at the end". I have tremendous respect for him as a person and a player.
Nadal indicated that this knee problem is a flare-up of his existing issue, even though it had not been bothering him before Friday's match.
Del Potro advanced when Rafael Nadal retired after the second set with a knee injury. "I know what is going on with the knee".
The world number one had been targeting a fourth US Open title, but instead it's Del Potro who is eyeing his second after winning the title back in 2009.
"These moments are tough but I have to keep going and working hard to get more opportunities". "This time it was a little bit more aggressive with my movement", said Nadal. Lot of people in this room, including myself, would never have thought that at the age of 32 I will be here fighting for titles, fighting for the first positions of the rankings. "I am playing well".
'All my career everybody say that because of my style, I will have a short career.
Rafael Nadal talks with a trainer shortly before retiring before the third set of his U.S. Open semifinal against Juan Martin del Potro. Against Nishikori, he's 6-2, and 2-0 in Grand Slam play. "I had my biggest memories on the tennis court playing on this court ... but I was a kid".