The defending champion needed almost five hours to defeat ninth seed Dominic Thiem in a deciding tie-break after losing the opening set 6-0 and will face Juan Martin del Potro on Friday in a rematch of last year's semi-final.
Nadal leads 11-5, including the past three, each at a Grand Slam tournament: in the semifinals of the U.S. Open last year, followed by the semifinals of the French Open and a five-set thriller in the Wimbledon quarterfinals this year.
Thiem became the first player since Andy Roddick in 2004 to win a love set against Nadal in NY but the Spaniard once again dug deep to come out on top of a gruelling battle played in more punishing humidity.
The last time Nadal failed to get on the scoreboard in an opening set was against Philipp Kohlschreiber at the Miami Open a year ago, though he went on to win the other sets 6-2 and 6-3.
Naturally, Nadal was exhausted following the quarter-final win. The world No 1 and three-time victor at Flushing Meadows came within two points of victory, but Thiem wound up taking the fourth set on a tiebreak.
This was tennis at its absolute best, according to David Law (BBC 5 Live) and Catherine Whitaker (Amazon Prime Video), and also a brilliant advert for fifth set tiebreaks. Had Federer won that match and Nadal won tonight, the two would have met in the Semifinals.
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"He's maybe playing some of the best tennis ever right now for him", said the American. I had the passion to keep going always'.
Sevastova came into the match having broken her opponent's serve a tournament-leading 23 times. Of course, now I'm devastated a little bit.
The world number one came through a nerve-shredding sudden death tiebreak 7-5 to recover from a disastrous start and win 0-6 6-4 7-5 6-7 7-6 after four hours and 49 minutes of a baseline war.
"It's going to be stuck in my mind", he said. He's a great guy, a great player. That's one more today.
With a heat wave striking NY and temperatures crossing 27 degrees at night, it appeared that Nadal would see off the tie with a four-set victory but Thiem's seemed to have found his mojo once again.
There were no breaks of serve in the deciding set and a second tiebreak ensued.
The semi-final will pit Nadal against third seed Juan Martin del Potro for the second successive Grand Slam. Del Potro beat John Isner in a match that played out as most people expected. I didn't expect to receive a trophy after losing in the quarter-finals.
These two have faced one another 10 total includes including three times this year, though no previous meetings were during the US Open. Law and Whitaker say it feels like there's an irresistible force behind del Potro that's pushing him to the title.