Argentina's Juan Martin Del Potro felled Marin Cilic in a French Open clash of the titans, winning 7-6 (5) 5-7 6-3 7-5 on Thursday, and will now aim his heavy artillery at 10-times champion Rafael Nadal in the semi-finals.
Earlier Thiem needed to survive a nasty bout of the jitters to see off Marco Cecchinato and reach his first grand slam final.
Rafael Nadal extinguished the threat of Juan Martin del Potro to canter into another French Open final, where he will be seeking a record-extending 11th title.
The world No 1 faces Del Potro who has made the last-four for the first time in nine years.
Del Potro claimed the tiebreaker, and though he faltered late in the second set, he regained control of the match with five straight wins to claim the third. Del Potro has lost once in the French Open semifinal, which was in 2009.
Nadal will face Thiem in Sunday's final, which is Nadal's 11th at Roland Garros and Thiem's first in any major. However, I said that I'd be shocked if Cecchinato defeated Djokovic, and he managed that feat, so the Italian clay-courter can not be completely written off in advance.
Nadal fought them off - three at 1-1 and three more at 4-4 - then took the set with only his second break point when Del Potro slammed a backhand into the net. I ran to everywhere and I did - I mean, I'm finishing in good shape, which is good to me.
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Del Potro dipped at the start of the third and Cilic had points for a double break but the burly Argentine, roared on by chants of "Delpo Delpo" recovered his drive to hit back from 0-2 and take the third set.
Nadal edged a tight first set against Del Potro but then blew the fifth seed away.
"Today, all day, on Philippe Chatrier, the fans said "Forza, Marco", so I think this is the best moment for me", said Cecchinato.
"I played more aggressive", Nadal said, adding: "In my opinion, the match changed, no?"
If it's soggy and the court slows down, that will blunt some of Nadal's power and not allow his forehand to get as high on Thiem's one-hand backhand. "I saved two set points and luckily I won it".
Nadal's match against Schwartzman was delayed by rain Wednesday with the Spaniard up 5-3 and serving ahead 30-15 in the second set after dropping the first, ending his streak of 37 consecutive sets won at the Paris event.
Thiem is the only man to beat Nadal on clay in the last two seasons - in Madrid this year and Rome in 2017. "And in the end, I got it, and now I'm having a great present, looking forward for the future". Cecchinato was accused of match-fixing and suspended for 18 months in 2016, but he appealed, and his punishment was dropped on a technicality. "The big key was this set".