The winners of the men's and women's singles finals this weekend will both take home a tournament record €2.2m (£1.9m/$3.1m) for triumphing on the clay at Roland Garros.
That put Thiem, a 24-year-old from Austria who is seeded No. 7, in the semifinals at the French Open for the third consecutive year.
Nadal tightened his grip, going to 5-1 on another double break in the third set and sealed the match when Del Potro hit a backhand long.World 72 Cecchinato had knocked out three seeded players to reach the semi-finals, including 12-time major victor Novak Djokovic in the quarter-finals.But Thiem, 24, was too strong and now believes he has a particularly powerful weapon on his side for the final - breakfast.
"There is a reason why he won 11 times here", said Thiem, a 24-year-old Austrian appearing in his first major final.
Rafael Nadal's remarkable Roland Garros reign continued as the world number one clinched an 11th French Open title with a straight-sets win over Dominic Thiem on Sunday.
Once Nadal stole the opener with a break in the 10th game, it was one-way traffic as he left Del Potro trailing in a cloud of Parisian clay dust to set up a final against Dominic Thiem.
"Thiem beat me in Madrid this year and played with big power". Those conditions might have contributed to the cramping that affected Nadal about two hours into the final, at 2-1 in the third set. "I never dreamed in 2005 when I played my first final here that I would be in another one".
"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.
"For me, the motivation to play here is always high, as high as possible", he said.
Nadal beat Stan Wawrinka in the 2017 French Open.
"But I have a plan". This year they have met twice and split the wins, with Nadal destroying Thiem 6-0 6-2 in Monte Carlo and Thiem winning 7-5 6-3 in Madrid. If Thiem can not find the court enough with his attempted winners, then the match will swing in Nadal's favour. So when I had the chances, I just tried to convert.
Nadal will retain the world number one ranking if he wins - but that will be the last thing on his mind.
At least Thiem has in the past found the key to unravelling the Nadal clay court enigma. If that gave him hope ahead of their fourth meeting of 2017 in the French Open semifinals, it was quickly dispelled in a 3-6 4-6 0-6 loss.
"If I want to beat him", Thiem said, "I have to play that way".
Siniakova was the doubles runner-up at last year's U.S. Open with a different partner.
"It was a very hard first set and Juan Martin had lots of opportunities".
"Not a very nice feeling", Thiem said.
At long last, Thiem converted his fifth set point when Cecchinato sent a forehand long.
He hit a forehand long for 0-30 and then hit another forehand into the net to gift Nadal three set points.