Asked whether his five-setter with Thiem may have had an impact on his knee, Nadal replied: "You never know".
Paul Annacone, who coached Federer for years and works as an analyst for Tennis Channel, told ESPN.com, "I don't think Delpo's backhand is strong enough to be a weapon against Rafa, but it's definitely less of a liability than it was a year ago".
"I think it was 2-2 in the first, 15-0, that I felt it". Nadal was done. And he was done in, as much by his troublesome right knee as Del Potro's heavy, forceful shots.
Djokovic said: "I personally like him very much, not just as a player but as a person". But no, it was not the day. "But at the same time, I felt like the six months off served me very well to find new motivation, inspiration, to recharge my batteries, and also to understand how I want to continue playing tennis, in which way, whether there are certain changes that I want to make with my racket and my game itself", he continued.
"That was not a tennis match at the end, no?"
He needed his right knee strapped after the seventh game before gallantly fighting off two set points in the 10th game, breaking Del Potro who was serving for the set.
Del Potro, who lost to Nadal in last year's semi-finals, said he was now focused on extending his career, sticking to a daily yoga regime and spending hours on treatments for his wrist and body.
"One of the keys of the match will be return, how well can I return, how many returns I can get back in play, but also try to have some depth in that return, and how accurately I can serve myself". Yeah, I mean, I think I have two finals won and five losses. This is my favourite tournament, my biggest memories on a tennis court came on this court in 2009.
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Nadal was devastated by having to pull out although he believes he hasn't suffered any long term damage.
"The problem is this time it was something a little bit more aggressive because it was in one movement".
As everybody knows, this is my favorite tournament since I'm kid.
As Nadal himself noted during a news conference that closed as he began to choke up, this was the second of those defeats that involved him quitting during a grand slam match because of an injury; the other was in the fifth set of his Australian Open quarterfinal.
"It's not about losing, it's about not having the chance to fight for it", he said.
Twice champion Djokovic, who reached his eighth final at Flushing Meadows and is seeded sixth this year, broke Nishikori in his first service game and closed out the opening set with an ace in 37 minutes without facing a break point.
This has been the Serbian's most consistent slam - aside from previous year, when he was sidelined by an elbow problem, he has not lost before the semi-finals since 2006 - but he has only won the title twice.
A nasty ankle roll late in the first set had fans fearing Nishikori mighty be forced out of his semi.