Both men advanced by winning quarterfinals suspended the night before because of rain.
"That", del Potro would say later, "was my chance".
He did not look unduly affected though and his heavy groundstrokes took a toll on Nadal who rescued three more break points in the ninth game.
Argentine Del Potro often had Nadal in trouble in a ferocious first set but paid heavily for failing to convert any of the six break points that came his way at 1-1 and 4-4.
However, the 32-year-old, who has won 16 Grand Slam titles, including 10 on the famous red clay, showed no signs of weakness when play resumed on Court Philippe Chatrier as he grabbed 12 of the first 13 points to lead by a break after he capped the second set.
The 24-year-old Austrian, widely considered the second-best player on clay right now, has a decent record against Nadal and, for the second year in a row, he is the only player to have beaten the Spaniard on clay coming into Roland Garros, after snapping his run of 50 sets in a row when they met in the quarter-finals in Madrid last month.
"I played more aggressive", Nadal said, adding: "In my opinion, the match changed, no?"
"It's not easy. I've been on court just a couple of minutes ago".
Although at times during the first set against Del Potro, especially when he was 0-40 down at 1-1 and 15-40 down at 4-4 it looked as though he might be in for a real scrap. That crucial early juncture altered the course of things, though.
"It was really important for my mind to know I could win against a great opponent".
"Sometimes he gets fired up but has this calm about him that I think worries Nadal and Federer". If he wins the game, OK.
"I am a human person", Nadal said following his 4-6 6-3 6-2 6-2 victory. The Austrian reached the semifinal stage at this competition past year but is confident of going one better this time.
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And the first set was indeed tight, even with del Potro calling for the trainer at 2-3. As long as Thiem continues to serve at such a high level, he's going to give himself a fair chance at the biggest victory of his career.
"He's the big favourite against everybody and still I know how to play against him", Thiem said. "It's so good for me, for my team, for my family".
Too good for almost everyone, nearly every time, on clay.
They tried their best to rouse their man in the third set but it was mission impossible as Nadal, clubbing the ball as if his life depended on it, finished like a raging bull as he headed into his 24th Grand Slam final.
He beat the Spaniard in the Madrid quarter-finals this year having won in the Rome last-eight in 2017.
So what will he need to do to counter the credible threat of Thiem, the only man to beat him on clay this year, on Sunday?
Nadal has lost French Open defeat only twice in his career and will be strong favorite to claim the title.
In Saturday's women's final, number one Simona Halep of Romania will face number 10 Sloane Stephens of the United States.