Nadal claimed his sixth French Open to equal the record of Borg, also taking his Slam total into double figures at 10.
Thiem is the only player to beat Nadal on red clay over the past two seasons.
Nadal, a 16-time Grand Slam champion, claimed his 85th win in Paris 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.
"But I believe I will be ready for the final".
"I couldn't play my best because of him", said del Potro, the 2009 U.S. Open champion. So there is not 10 more chances to keep playing here. But I get closer to them, just asking who was it?
Clay specialist Thiem made the last four at Roland Garros in both 2016 and 2017 but fell short against eventual winners Novak Djokovic and Nadal, continuing his wait for a first grand slam final.
Nadal and Thiem will face off on Sunday at a 9 a.m. ET to close the French Open.
"Sometimes he gets fired up but has this calm about him that I think worries Nadal and Federer".
Moya says he'll make sure they try to address that ahead of the final.
Nadal seldom leaves the door cracked very long. After he held there, that was pretty much that for del Potro. Nadal thrashed the Argentine to win the match in straight sets: 6-4,6-1,6-2.
At long last, Thiem converted his fifth set point when Cecchinato sent a forehand long.
"Sometimes you play better, sometimes you are more nervous".
"He said "I'm a human man, a human person and I get nervous". He's playing with big confidence.
In the first all-American Roland Garros semi-final since 2002, the 10th-seeded Stephens stayed composed throughout as Keys, who has not won a set against her in three meetings, peppered the court with unforced errors.
His decision to enter a smaller tournament in Lyon, France, the week before Roland Garros - an event he won - doesn't appear to have sapped him.
Both of Thiem's victories came in three-set matches in Rome and Madrid, but sustaining it over five sets in Paris is likely to be a different proposition.
Nadal has been untroubled en route to the semis, apart from a blip in the quarter-finals against Diego Schwartzman, but the towering Del Potro looms large on Court Philippe Chatrier on Friday. The second set like, the first was dominated by serve until Cilic fired himself up to break at 4-4 - a furious Del Potro throwing his racket at the changeover.
Federer, who has skipped the French Open, has reached 11 Wimbledon finals, winning eight.
"He is, for me, the best competitor I ever saw in any sport, and I watch sport a lot for many, many years, " said Gunter Bresnik, Thiem's 57-year-old coach. Applying this to Sunday's final, we might expect to see three or four sets, but are less likely to see five. He is in the age of improvement. He played some unbelievable aggressive tennis to get through Djokovic in the quarters, just like he played fantastic aggressive tennis to get to the quarters.