The 16-year-old McNally, who is unseeded, beat Iga Swiatek of Poland 3-6, 7-6 (6), 6-4.
Rain still starring in the French Open!
The two remaining men's quarterfinals were suspended in progress and will resume Thursday.
Thiem, who beat Nadal in Madrid this season, Rome previous year and in Buenos Aires on clay in 2016, trails 3-6 in the pair's head-to-head.
"It is nearly impossible to beat Rafa", said Del Potro who had his chances in the semi-final but failed to convert any of his seven break points before his challenge petered out into tired hopelessness. "[Del Potro plays an] aggressive game from the end of the court, the baseline".
But those are not quite the same as the French Open, where Nadal is 86-2 for his career.
Nadal also survived multiple set points in the opener against Del Potro. Cilic is serving at love-30 in the 11th game.
"If I had won the second set, I think it would have been a totally different third set", he explained.
The 16th-seeded Gauff, who is 14 years old, beat 15th-seeded Leylah Annie Fernandez of Canada 6-4, 6-3.
The Argentine has never beaten Nadal on clay.
Thiem saved a first breakpoint in that game but couldn't save the second, hitting a forehand wide.
A few hours earlier, as Nadal and Thiem warmed up, the booming voice of the announcer at Court Philippe Chatrier detailed the bona fides of both.
Thiem survived two tough sets against Marco Cecchinato before cruising through the third for a 7-5, 7-6, 6-1 success. Last year, he took the trophy without dropping a set.
Rafael Nadal briefly interrupted the French Open final against Dominic Thiem in the third set with an apparent problem with his left wrist. He will now be looking to win a record 11th grand slam title of the tournament.
Nadal won the first two sets. "I never dreamed in 2005 when I played my first final here that I would be in another one". But I know how to play against him. She led Jelena Ostapenko by a set and 3-0 in last year's final at Roland Garros.
Stephens and Halep will face each other in the final in Paris on Saturday.
Del Potro fought back tears during his post-match interview and was loudly cheered by fans at Court Suzanne Lenglen.
Otherwise, he stayed steady in a 7-6 (5), 5-7, 6-3, 7-5 win over No. 3 Marin Cilic in a matchup between two past U.S. Open champions.
Thiem is also the first Austrian to reach a grand slam final at Roland Garros since Thomas Muster's victory in 1995.
She needed only 70 minutes to eliminate Sharapova and dominated her with superb returning, breaking the former No. 1 a half-dozen times.
But the Spaniard was impressive on Friday‚ finishing with 35 winners and 19 unforced errors to Del Potro's analysis of 20 and 32.
A visibly dejected Schwartzman took only one point on his serve before losing the fifth game while Nadal held to lead 5-1.
"If you tell me seven, eight years ago that I will be here. having this trophy with me again, I will tell you that is something nearly impossible", Nadal said.
Muguruza, a two-time major champion, entered the semifinals having not lost a set in the tournament.
That the second seed, Alexander Zverev, reached his first grand slam quarter-final this week says it all.
"I'm feeling so, so happy to make the right decision to play here".
Nadal defeated del Potro in four sets the last they met - US Open 2017 semi-final.
It made no difference; he could probably win this with one hand tied behind his back.
Thiem is contesting his first Grand Slam final.