But as the Spaniard struggled to win points on his first serve, he was broken again for 5-4 and Schwartzman served for the set.
The 11th-seeded Schwartzman appeared to have done his homework well as he dominated the early exchanges to bag the opening set: an achievement in itself as it was the first one Nadal had lost at Roland Garros since 2015.
The 16-year-old McNally, who is unseeded, beat Iga Swiatek of Poland 3-6, 7-6 (6), 6-4.
The battle wore on and on, before, on the Austrian's fifth set point, Cecchinato went long to fall two sets behind.
"The second-set tiebreak was the big key to the match, 100 per cent, because obviously he felt all the matches from these two weeks after that", the seventh seed said. Schwartzman saw two set points come and go but at the third time of asking, after a delay while a spectator was helped out after feeling ill, the Argentine ripped a forehand victor. And he set up a French Open final that, pardon the cliché, is a worthy clash between the king of clay and the man hailed as the prince.
Dabrowski also won the French Open mixed doubles previous year, but with Rohan Bopanna of India as her partner.
And the underdog soon faded as Thiem kicked on, racing to two swift breaks in the third and rescuing two break points to serve out the match in style.
But despite the straight-sets win, it wasn't a cakewalk for the Austrian.
"I am in a positive way always on court", Nadal said.
Only three times (once when he withdrew with an injury) has the Mallorcan not reached the French Open final since he first re-defined claycourt tennis in 2005 as a sleeveless warrior.
"I was a bit lucky", said Nadal, who is 10-0 in French Open semifinals and 10-0 in finals at the tournament. In the fourth game Friday, del Potro clutched at his left hip after being wrong-footed by one shot from Nadal and was visited by a doctor at the next changeover, but said afterward it was not a big deal.
"He is a nightmare opponent for Nadal", three-times French champion Wilander, hosting Eurosport's flagship Game, Schett and Mats programme at Roland Garros, told Reuters.
"It may sound easy and logical, but I don't want it to be. Facing Rafa, I'm not the one who has the pressure".
The world number one is still the overwhelming favorite to pick up his 11th French Open title, but former victor Mats Wilander thinks Juan Martin del Potro, Nadal's semi-final opponent could cause him some troubles.
Nadal lost the first set to his opponent and things were level in the second when the match was suspended last night because of rain. All have been on clay, and Nadal leads 6-3, including 2-0 in Paris, beating Thiem in the first round in 2014 and the semifinals a year ago. At No. 72, he is the lowest-ranked French Open semifinalist since 1999.
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. Coincidentally, the only time he has looked in any danger was on day 11 when Diego Schwartzman, the 11th seed, took the only set of the tournament off him.
While a player's Grand Slam record is what is talked about most, the debate for the greatest single-surface player would be rendered incomplete without a look at their performance on the surface across tournaments.
Simona Halep celebrates her victory against Garbine Muguruza.