A trainer was called and he briefly inspected Nadal's left wrist.
He broke the Austrian in the third game of the set.
"Thiem is an animal physically and in terms of hitting the ball hard on the forehand".
And the first set was indeed tight, even with del Potro calling for the trainer at 2-3.
But Austrian Thiem will see this as a fantastic opportunity to reach a maiden Grand Slam final, after losing to Djokovic and Nadal at the semi-final stage in each of the last two years in Paris.
Until 4-all, 15-all in the opening set, to be precise. But Nadal restored his authority with a 7-5 6-3 win in Monte Carlo later that year. Since then, the Spaniard has been imperious.
Thiem‚ who was playing in his third successive French Open semi-final‚ admitted the second set had been a nervy encounter after needing five set points to get through it.
Thiem is contesting his first Grand Slam final.
However, he quickly recovered and looked to have the ascendancy, but the Argentine let six break-point chances slip and Nadal pounced with a solitary break in the 10th game which was enough to allow him to seal the set after 54 minutes of high-octane action.
He led by two sets plus a break early in the third, when the middle finger on his racket-wielding left hand was cramping so badly he couldn't straighten it.
Thiem is the only man to defeat 10-time French Open champion Nadal on clay this season.
Nadal had 12 unforced errors.
However, he reclaimed the break in the eighth game before edging ahead to 5-4.
Increasingly frustrated at Cecchinato's resistance, Thiem lost his composure completely in the second set.
Rafael Nadal admits that his desire to clinch an 11th French Open title on Sunday is fired by his growing realisation that the end of his career may not be too far away.
He saved three break points before taking the game and set 6-2.
Thiem, 24, is showing no signs of nerves in his first Grand Slam final. "Today was unbelievable for me, with all of Philippe-Chatrier Court full".
The crowd cheered when Zidane was shown on the big screen.
He still occasionally inconvenienced the reigning champion, not least when a deft drop shot brought up a point to break back at 2-4.
Marco Cecchinato of Italy was defeated by Dominic Thiem of Austria during day thirteen of the 2018 French Open. At No. 72, he is the lowest-ranked French Open semifinalist since 1999.
The 16th-seeded Gauff, who is 14 years old, beat 15th-seeded Leylah Annie Fernandez of Canada 6-4, 6-3.
Krejcikova and Siniakova, who are both 22, were playing in their fifth major tournament together.
The sixth-seeded Frenchmen have won two Grand Slams together.
They knocked out the top-seeded pair of Timea Babos and Kristina Mladenovic in the quarterfinals.
The winners of the men's and women's singles finals this weekend will both take home a tournament record €2.2m (£1.9m/$3.1m) for triumphing on the clay at Roland Garros. The world No. 1 became the second man in the Open Era to make 11 finals at the same major, tying friendly rival Roger Federer's mark on the grass of the All England Club.
Del Potro had required the doctor early in the first set after appearing to jar his ankle in the red clay.