The world number two led 4-0 in the first set and 5-1 in the second, but Argentinian 20th seed Schwartzman came back to level both.
Nadal’s forehand seemed to desert him at times and he required treatment on his left forearm, a worry given that injuries have already accounted for, or contributed to, the demise of Novak Djokovic and Roger Federer.
As the last of the ‘big three’ still standing, Nadal knows he has a golden opportunity to take a huge step towards overhauling Federer’s record of 20 grand slam titles.
He will be one behind the Swiss great, and three clear of Djokovic, if he completes the job here on Sunday.
And while at 38, Federer’s chances of adding to his tally are surely waning, the 33-year-old Nadal, fitness permitting, will almost inevitably collect a 13th French Open title next year.
When Nadal raced two breaks ahead in the opening set, he could hardly have imagined it would take the best part of an hour to claim it. And the pattern was repeated in the second.
Nevertheless, Nadal wrapped up 6-4 7-5 6-2 victory to set up a semi-final against Italian 24th seed Matteo Berrettini.
“It was straight sets but a big challenge,” Nadal said. “To see myself in the situation to lose both breaks in a row was tough.
“But I’m super happy with the way I accepted the challenge. Here I am, semi-finals, it means everything to me.
“Physically I’m fine. Today was a heavy day, big humidity, and I sweat a lot. I had some big moments at the end of the second set and beginning of the third, but the last few games I felt good again.
“Berrettini? I saw him play. He has big serves, big forehands, and is super confident. He makes big steps forward every week so it will be a big challenge. But now is the moment to keep going.”
Earlier 23-year-old Berrettini beat 13th seed Gael Monfils in the match of the tournament so far, finally sealing victory in a fifth-set tiebreak, 3-6 6-3 6-2 3-6 7-6 (5).
Berrettini needed five match points to fall over the line, having double-faulted with his first at 5-3 in the fifth.
It became a mental battle with an exhausted Monfils also double-faulting time after time at the climax.
His fifth double fault, in the tiebreaker, gave Berrettini two serves to win it, but Monfils passed him to haul one mini-break back, and the Italian netted the second.
But Berrettini came out on top in a brutal rally to bring up two more match points.
He was aced with the first, but when Monfils hit a service return long, Berrettini dropped to the floor on the baseline in disbelief.
Berrettini said: “Right now I can’t remember any points, just the match point – and the double fault!
“When I was playing I was thinking it was one of the best matches I had ever seen. I was playing, but I was watching, too. I’m really happy.”
Johanna Konta insists she is capable of winning a grand slam title as long as she keeps giving herself the chance.
The British number one has enjoyed a fine season in the majors, with a semi-final at the French Open followed by a quarter-final at Wimbledon, and culminating in a first appearance in the last eight at the US Open.
However, that run at Flushing Meadows was ended by Elina Svitolina, the fifth seed, 6-4, 6-4 on Arthur Ashe.
Konta was attempting to become the first British woman to reach the semi-finals in New York since Jo Durie in 1983.
But she fell short against a familiar foe, Svitolina extending her 100 per cent record against Konta to five matches.
“I do feel that was the probably the best I felt her play against me. She played so well, to be honest,” said Konta.
“Actually, I didn’t play badly at all. I actually felt like I was doing a lot of good things out there, a lot of the right things.
“She just made me play that extra ball. It’s frustrating. You know, I would have loved to have come through that and come through a challenge like her, but I guess it will just have to be next time.
“I definitely think it’s just the more opportunities I give myself like this, the more chances I have of going a step further and even more steps further.
“I don’t have any regrets or any hindsight in what I did before or during the match. I think I did a lot of good things.
“I think I can definitely still get better and better, especially against an opponent like her. I can take a lot of good things from this still.”
Konta had been streets ahead of the rest of the field with the amount of winners she has hit this tournament and she had another 24 here.
But that was offset by an unforced error count of 35, compared to just 13 from the metronomic Svitolina.
The Ukrainian made the first breakthrough in a hard-fought fifth game, Konta eventually succumbing with a backhand into the net.
Konta, with actor Tom Hiddleston watching from her box again, broke straight back. But she then promptly gave away break point with a misguided drop shot and Svitolina pounced again, arrowing a backhand into the corner.
Back came Konta with another break point, but it was repelled with an ace as Svitolina clinched the opening set.
Another unforced error handed Svitolina a break for 3-2 in the second as a Konta forehand drifted wide.
Konta immediately hit back for 3-3, a stunning drop shot flopping over the net just out of Svitolina’s reach, only to then find herself at 0-40 and, when a volley fell short, she was passed and broken once more.
Konta faced two match points on her own serve, and gamely saved both to force the ball back into Svitolina’s hand, but Svitolina converted number three.
For Konta, it is now a trip to Broadway to watch Hiddleston in ‘Betrayal’. For Svitonina, it is back-to-back major semi-final appearances, having also reached the last four at Wimbledon.
The 24-year-old said: “I think we were both striking the ball well, it was quite even. I was just taking one point at a time and trying to stay calm.
“I was relieved I could serve it out and just very happy with the way I handled the pressure.”
Fast and furious Serena Williams dropped just one game as she blew away China’s Wang Qiang 6-1, 6-0 at the US Open quarter-finals.
Wang, seeded 18th, did not manage to hit a single winner as she was taken apart in her first grand slam quarter-final.
It was the American’s 100th career victory at the US Open and at 44 minutes was the quickest match at this year’s tournament.
“It’s really unbelievable,” said Williams, 38. “When I first started here I was like, 16. I never thought I would get to 100 wins and still be here but I love what I do.
“When I’m playing someone I know is playing well I know I have to step up or go home, and I want to stay here.”
Williams showed no ill-effects from rolling her ankle during her fourth-round win over Petra Martic.
“I’m feeling good,” she added. “It’s been a tough year but physically I’m feeling great and more than anything I’m just having fun when I come out here.”
Williams, the eighth seed, will play Elina Svitolina in the semi-finals.
“She’s had a great year,” said Williams. “She made the semis at Wimbledon and I feel she wants to go one step further, so I feel I’ll have to come out and play really well.”
Svitolina, the fifth seed from Ukraine, beat Britain’s Johanna Konta 6-4, 6-4 over 100 attritional minutes.
She said: “I don’t think that I was expecting going into this tournament that I’m going to make semis.
“I’m just taking one match at a time. It’s a grand slam. There is lots of pressure, lots of expectations from people. But you also put lots of pressure on yourself.
“You have to just go out there, do your job and don’t think too much what’s going on around you, and who handle it the best wins.
“Definitely it’s a big challenge to play against her. Obviously Serena is amazing champion. It’s probably it’s going to be really tough against her.”