On Friday as well, he held his nerves at the crutch moment - when he faced three set points in the second set tiebreak and worked his way back from fudging his own share of five set points.
Thiem's ability to bully opponents from the baseline was on display in his 6-4, 6-2, 6-1 victory over No. 2 seed Alexander Zverev in the quarter-finals.
"I have a plan", he said. "That's why he's the No. 1 and beating all the guys", Del Potro said. That crucial early juncture altered the course of things, though. "And I will try just to be calm during the match and try everything I can".
'Still, I know how to play against him. If he wins the game, OK. "We don't have to forget that, we have to be aware of that, but also not give too much importance to that otherwise it can get in your head and it's not helping".
Truth is, there were not many the rest of the way, as Nadal finished with 35 winners and just 19 unforced errors.
Friday will be the second time Nadal and Del Potro meet at Roland Garros.
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).
"I think he's going to be unbelievably nervous Nadal", Whitaker said on the Tennis Podcast.
Too good for almost everyone, nearly every time, on clay. It is his best result at Roland Garros since 2009 when he also made the semi-finals losing to Roger Federer in five sets. We have officially run out of superlatives to describe this man's clay court greatness, and we've also run out of intra-tennis comparisons.
"If I want to beat him", Thiem said, "I have to play that way".
It is the second straight all-American girls' final at Roland Garros, after Whitney Osuigwe beat Claire Liu a year ago.
Thiem, one of the most consistent performers on clay court over the last three seasons, conceded he is under a lot of pressure ahead of his first Grand Slam final, in which he will face the 10-time champion and Rafael Nadal.
Seventh seed Thiem is the first Austrian to make a final at the majors since Thomas Muster was champion in Paris in 1995. Of players under 29, only Thiem and Milos Raonic have reached major singles finals: Raonic lost in the 2016 Wimbledon final to Andy Murray.
Thiem weathered three set points for Cecchinato, at 7-6, 9-8 and 10-9.
By the absurd standards Nadal has set here, even facing those break points amount to a lapse, as does the opening set he dropped to the terrier-like Diego Schwartzman in the quarterfinals.
"It's his court, he has the keys and he has to take advantage of that", said Moya, referring to Nadal's track record at Philippe Chatrier stadium.