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.
Nadal dropped a set to the plucky Diego Schwartzman and looked close to dropping a set as Juan Martin Del Potro put him under pressure but stepped up the pressure himself and Thiem will find that what would normally be winners against any other player will come back time and time again.
Dominic Thiem is the iron man of men's tennis who is never happier than when his racquet is doing the talking.
His opponent Friday will be Juan Martin del Potro, who got choked up after waiting almost a decade to return to the final four in Paris - and dealing with three wrist operations in the interim.
There was a sense of inevitability in the air when Del Potro served at 4-5 and sure enough Nadal turned the screws to grab two set points at 15-40.
And the first set was indeed tight, even with del Potro calling for the trainer at 2-3.
"I'm really, really happy that I won this match", said Halep, who lost to Jelena Ostapenko in the final previous year. Del Potro had forced six break points during the first set but Nadal fended each one off.
He buckled at 5-5 though and sent a backhand long on break point, leaving Del Potro to serve for the match, which he did easily as Cilic sent three more backhands out. "Gave me time for reflection, to calm my nerves and to see how I needed to change things".
"He's one of the best players in the world on this surface", Nadal told reporters.More news: David Beckham And Victoria Beckham Say The Divorce Rumors Are A Joke
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Despite some spirited resistance, the 11th seed couldn't live with Nadal's new-found intensity and revamped strategy, and the 16-time grand slam champion sealed the victory for the loss of only four games with his fourth match point. 'If I want to beat him, I have to play that way like I did in Rome and in Madrid.
Thiem had three set points at 6-3 but Cecchinato quashed them all and reversed a triple of set points in his favor, but he couldn't convert a tiebreaker win this time as Thiem moved ahead after two errors.
While we have an edge here, I appreciate that backing a heavy odds-on shot isn't most people's cup of tea, so the game handicap should be a viable proposition.
"What will be important will be to try and make him play in uncomfortable situations", said Moya, the 1998 French Open champion.
"But I have a plan". "Today in the first set, for example, I was not playing great, was suffering because he was playing better than me".
This included a quarter-final success in Madrid last month.
But while the Swiss has succumbed in three title matches in southwest London, Nadal has never tasted defeat in a final in southwest Paris, as Bresnik - and the rest of the tennis world - knows. Thiem battled to a 7-5 7-6 and then walloped a clearly dejected Cecchinato 6-1 in the third to qualify for the final after making the semi-final on three consecutive occasions. "It's so good for me, for my team, for my family". Cecchinato was accused of match-fixing and suspended for 18 months in 2016, but he appealed, and his punishment was dropped on a technicality.
"When I missed that volley it wasn't a very nice feeling".
What went through Nadal's mind right then?