Nadal, who was on court for four hours and 48 minutes in a tie which finished at 2.04am local time in NY (7.04am BST), commented on the humidity of the playing conditions that has troubled so many players in this year's Open, and broke down his performance.
Thiem levelled the match with a fourth set tie-break victory before Nadal finally edged a tense final set after another tie-break. And it was the way Nadal won - with power and tireless energy, matching his younger opponent's athleticism and appearing to grow stronger with each monstrous forehand - that told the story of how the two best players in the world have fared in recent months.
Tuesday's first set between No. 1 Rafael Nadal and No. 9 Dominic Thiem was a stunner.
Thiem produced a stunning first-set performance to take the opener without losing a game but fell victim to a Nadal fightback.
But Nadal held on and is into the semifinals at a third consecutive major tournament, as he bids for a fourth US Open title and 18th Grand Slam trophy overall.It was a question of a little bit of luck at the end.
"It's been a great battle", an exhausted Nadal said. The World No.1, for the first time, started the match with such a worst beginning. While Thiem clearly made the big winners, Nadal just knew when to raise his level by just a solitary notch, that awarded him the win.
Nadal jumped over the net to console his Austrian opponent at the end, the pair embracing as those left inside Arthur Ashe Stadium rose to their feet to give them a thunderous ovation.
"I mean, he's a great guy".
"That's what I was preparing for", Thiem said.
"Whatever the humidity is on outer courts or in the city, I think it's amplified on center court", Isner said.
Despite that, the 29-year-old was never broken in the 3 hour 31 minute match where Isner unleashed 26 aces but was undone by 52 unforced errors compared to Del Potro's 14. I tried to stay in the match some way in the second set.
"It has been physically demanding because the conditions have been tough out there, too. Forever I'm going to remember this match, for sure", said Thiem after his first ever hard court match against Nadal. Nadal was once again able to recover after going down a break, forcing the set to a tie-breaker.
"He will say "this is my opportunity and I'm expected to lose, but I have to go out there guns blazing".
"To play John in these kind of matches, it's like an epic". Nadal then broke again to take the set 7-5.