Nadal, bidding for a fourth title in NY and 18th major, had spent the best part of 16 hours getting to the semi-finals.
When Rafael Nadal finally finished a almost five-hour climb into the U.S. Open semi-finals, he thought backward as much as ahead.
The top seed and three-time champion Nadal was already struggling with his right knee and had to have it taped up in the opening set - a sign of things to come.
The women's semi-finals were on Thursday night, with Serena Williams facing Anastasjia Sevastova and Madison Keys facing Noami Osaka.
On Sunday, the powerful Argentine with the massive forehand that zips along at more than 100 miles per hour (160 kph) will face two-time U.S. Open champion Novak Djokovic or 2014 runner-up Kei Nishikori. She was the youngest player to ever reach the girls' singles final at the U.S. Open a year ago before falling to fellow American Amanda Anisimova.
All that was certain at that point was that his bid for a fourth US Open title, and second in a row, was gone.
RAFAEL NADAL: I can't say that I am happy because I am not.
In Friday's other semi-final, Djokovic will try to extend the form that has seen him win 24 of his last 26 matches, an impressive run that includes titles at Wimbledon and the US Open tune-up event in Cincinnati. "Even before I went through the whole process of surgery, post-surgery recovery, I was still feeling for players that went through injuries", Djokovic said.
Del Potro, 29, is in his second U.S. Open final nine years after reaching his first.
Rafael Nadal retired from his semifinals match against Juan Martin Del Potro after two sets on Friday.
For del Potro, it was an odd way to return to an important summit. In the months that followed, he was briefly haunted by the old spectre of injuries again - he retired in Rome with a groin strain, withdrew from Queen's with an adductor injury and missed the Rogers Cup due a left wrist injury.
Now, the Argentine has regained strength there, allowing him to change it up from slice to power. I am very competitive at the age of 32.
The love affair with NY runs both ways. As everybody knows, I had an issue, it was in the second or third match.
After Del Potro won the set, Nadal took off his headband, sat down and pulled off his wristbands while a trainer spoke to him.
In front of him on Sunday is a man who has beaten him 14 times in 18 contests and in all four of their slam matches. He, Marin Cilic and Stan Wawrinka are the only players outside of tennis's "Big Four" to have won a major singles championship since the 2015 French Open.
This one, given the added difficulty created by unusually high heat and humidity, would certainly be meaningful. He couldn't defend his title in 2010 with a wrist injury, that would go on to require four surgeries, change his style of play, make him nearly give up the sport and define his career for the next eight years.
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