On Friday, del Potro was by far the better player in the first set while, in the quarter-finals, Diego Schwartzman won the first set.
"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.
Nadal, who has beaten Thiem in six of their nine meetings, will start the final as the favourite.
The Spaniard required barely two hours to win 6-4, 6-1, 6-2. Meanwhile Austria's Dominic Thiem is bidding to reach his maiden Grand Slam final against surprise semifinalist Marco Cecchinato of Italy. I know I have to play at my best.
"I have a plan", he said.
Del Potro saved one but a backhand error handed Nadal the set and he never took a backward glance after that.
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"Just thinking in a positive way and just thinking that I have to hold. I think after that I changed, I increased the intensity, increased the determination on how to play my shots, so the match changed drastically". If he wins the game, OK. "I don't feel the pressure now in semifinals", Nadal said. "I was so nervous before the match because I knew it would be a tough match".
Schwartzman compiled a 20-4 edge in winners for the set, closing it with a forehand that landed right at the baseline.
Nadal did not have almost as hard a time against the fifth-seeded del Potro.
"It's been a lot of years of playing and doing the same things every year".
At the press conference following the game, Nadal was asked what "biological age" he was at after a study which showed Real Madrid superstar Cristiano Ronaldo was just 23 (the footballer is actually ten years older than that).
"Today, all day, on Philippe Chatrier, the fans said, "Forza, Marco", so this is the best moment for me", he added.
"He's one of the best players in the world on this surface", Nadal told reporters.
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At least Thiem has in the past found the key to unravelling the Nadal clay court enigma.
"Forget the 10 (French Open titles) and forget what happened previous year, Thiem has to remember what happened in Madrid which is a title Nadal wants to win nearly as much as the French", Wilander, Eurosport's chief analyst, told Reuters.
"I have to play that way like I did in Rome and in Madrid", Thiem said.
The No. 1 seed could scarcely have achieved more at the French Open-he has played 87 career matches there and lost just two-but he still has the hunger for more success.
In Saturday's women's final, No. 1 Simona Halep of Romania will face No. 10 Sloane Stephens of the United States.
Thiem, meanwhile, becomes the first Austrian to reach a Grand Slam final since Thomas Muster triumphed at Roland Garros in 1995.
No player in history has won as many grand slam titles at a single tournament as Nadal has in Paris.
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Thiem had to weather three set points for Cecchinato, at 7-6, 9-8 and 10-9. Then Thiem got to 10-all with a drop shot of his own.