Rafael Nadal acknowledged he was fortunate to claim a thrilling US Open quarter-final victory over Dominic Thiem, who he conceded did everything possible to claim victory.

Nadal recovered from losing the first set without winning a game to win 0-6 6-4 7-5 6-7 (4-7) 7-6 (7-5) in a gruelling four hour and-49-minute epic.

The defending champion was a break down at 5-4 but fought back to move two sets to one up and, though he was unable to close the match out in the fourth, he did so in the fifth-set tie-break, setting up an enticing encounter with Juan Martin del Potro in the semi-finals.

"Very demanding in all aspects," Nadal said of his triumph at a post-match media conference. "Have been a very tough start for me. After that first set, then the match became more normal. Tough match against a great opponent.

"The conditions have been tough out there, too. Not crazy, but tough. And, yes, I played against a great opponent. He played a good match. [I am] sorry for him. He's a top guy. He's one of the best guys on tour. He always plays and practices with a great attitude.

"[I am] sad for him because when [he arrived at] this moment, he did all the things well to win the match. Me, too, I think. I fought until the end. Yeah, it was [a] question of little bit of luck at the end. Happy, of course, to be in semi-finals again, it means a lot to me. It's good to have now two days [without a match] that probably give me the chance to be 100 per cent in the semi-finals."

Nadal holds an 11-5 record against Del Potro. He overcame the Argentine at the same stage at Flushing Meadows last year before doing so in the last four at this year's French Open and then winning a five-set encounter in the Wimbledon quarter-finals.

"It will be a very tough one," Nadal added of his last-four tie. "He's a great player on grass. Well, he's a great player everywhere. But the challenge of playing him on hard of course is even higher for me personally than playing against him on clay.

"It will be a big challenge. It's a match that we know each other very well. I know he's playing well. I know that I will have to play at my highest level to keep having chances of success. I am focused on trying to make that happen.

"I think in the last three sets I improved. I increased my level. It was a step forward for me in the dynamic of the tournament. I hope to continue that way."