When you're in a final like Roland Garros, it's a great happiness.
Nadal, a 16-time Grand Slam champion, claimed his 85th win here against just two losses after weathering a big-hitting opening from 2009 US Open victor Del Potro to win 6-4, 6-1, 6-2.
The 24-year-old Thiem, the youngest man since Nadal to reach the final in Paris, ended the contest on his first match point with a forehand victor.
It was hard not to feel sympathy for Thiem, playing in his first grand slam final against a man who simply does not lose this particular battle.
Going into the final at Roland Garros, Thiem's prospects of winning his first grand slam will be boosted as he is the only player to beat Nadal on clay twice (in 2017 and 2018).
In Sunday's final, Thiem will face either No. 1 Nadal or No. 5 Juan Martin del Potro.
Play has since resumed and Nadal now leads 4-2 in the third set.
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 quarterfinals. After breaking Nadal's serve to take the ninth game, Schwartzman hit a forehand victor at 40-30 to take the first set 6-4. "So I need more work for go top 10 or top 20, so these matches was very important for me".
Seventh seed Thiem is the only player to have beaten Nadal on clay in the past two years, and he claimed he had a plan to thwart the Spaniard in Paris.
"I have done a lot of things well in the clay court season, but I believe I can be ready for the final".
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Del Potro had required the doctor early in the first set after appearing to jar his ankle in the red clay.
He added: "For me, the level is very good at the moment". Although he lost the first point of the fourth game, a flawless drop shot got Schwartzman to 15-all, and a backhand victor from followed by a drop shot gave Nadal a second break an 4-1 lead. I know I have to play my best if I want to have chances.
Del Potro had required treatment on a hip problem suffered early in the first set and he visibly wilted in the second, raising his arms in mock celebration when he won a solitary game while already 5-0 behind.
Cecchinato had never won a Grand Slam match before this year's Roland Garros but made the semis with a string of stunning wins.
"Forget the 10 (French Open titles) and forget what happened past 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.
Nadal said of his cramp: "I was very, very scared".
In Madrid, Thiem said the key was to be positive against Nadal.
Thiem had to weather three set points for Cecchinato, at 7-6, 9-8 and 10-9.
"If I'd won the second set I think it is totally different, the third set", he said.
In the third game, Schwartzman missed three points in a row and then lost the game when he missed a drop attempt. Cecchinato was accused of match-fixing and suspended for 18 months in 2016, but he appealed, and his punishment was dropped on a technicality.
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Most recently, Cape May prosecutors have said they will not press charges against the police officers involved in the video. Weinman insisted in a since-deleted Facebook post and in the video that she had not had anything to drink. "Kind of upset.