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.”
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