Nadal crushed Del Potro's spirit with the intensity of his tennis in the second set, bounding 5-0 ahead before the Argentine registered a game to lift the spirits of his fans.
'For me, the motivation to play here is always high, as high as possible, ' he said.
Cecchinato, who was banned from tennis in 2016 in a match-fixing investigation, had never won a grand slam match going into the 2018 event but dispatched eighth seed David Goffin and former world No. 1 Novak Djokovic in a string of surprise wins.
"You know when you start the clay-court season that Dominic, he's one of these players that have the chance to win every tournament that he's playing", Nadal said, "and maybe even (more so) here in Roland Garros, because he's strong physically. It was 6-4 and the one thing I wanted to do was win the set - and I missed an easy volley at one moment which was not a nice feeling".
Thiem is responsible for both of those losses - at Rome in May 2017, and at Madrid last month - which at least lends a little intrigue to Sunday's proceedings.
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Thiem is the only player to beat Nadal on red clay over the past two seasons. "I think he likes the conditions more here than in Madrid, for sure, and best of five is also a different story".
"I believe that there is chances, limited chances in your career", Nadal said. Although he came to life again in this tournament, he lost to the man from nowhere, Marco Cecchinato, ranked 72 in the world, in a 22-minute tie-break that catapulted the Italian into the semi-finals, where he ran out of steam against Thiem on Friday.
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. "I will try everything that my plan [is] also going to work out a little bit here and not only in Madrid or in Rome".
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"I have to find it inside me and find the right options for Sunday, the options that will be the right ones to succeed".
Yesh: It has been nearly a decade since del Potro made the semifinals here.
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So, the president , I can imagine him saying, 'We've agreed on this peace treaty, ' which is what we said in 2005. But I think it will take a little bit of time".
Both players contributed to an entertaining start, but Thiem quickly took control, breaking through in the opening game and then comfortably holding easily.
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.
"Del Potro has achieved great victories this year", said Nadal who dropped his first set at Roland Garros in three years in his quarter-final victory over Diego Schwartzman.
Cecchinato had never won a Grand Slam match before this year's Roland Garros but made the semis with a string of stunning wins.
A fourth set point for Thiem vanished because of one of Cecchinato's many drop shots.
Thiem sent Cecchinato scurrying around the court before breaking decisively for a 6-5 lead with a lovely forehand victor.
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They also stepped up Trump's assault on Trudeau himself - unprecedented in the neighbouring countries' longstanding relationship. They could also damage Japan and Germany. "If we're unable to make a deal we'll terminate Nafta", Trump said Friday morning.
Someone in the stands cried out during his serve, disrupting his focus as he double faulted and handed Cilic a critical break for 5-4 in the second set.