Nadal has never failed to win the title when reaching the last four at Roland Garros, but knows that Del Potro, who he has lost to five times in 14 meetings, is a risky opponent.
Cecchinato had never won a grand slam match until the French Open.
Thiem is looking to become only the second Austrian player - man or woman - to win a Grand Slam title after Thomas Muster's French Open victory in 1995.
To get to 11-0 in French Open finals, he'll need to get past the only man who has beaten him on red clay over the last two seasons.
Rafael Nadal powered his way into the French Open final and then claimed he can still move up another gear.
"I am sure you will win here in the next couple of years", Nadal told Thiem afterward.
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.
Both players contributed to an entertaining start, but Thiem quickly took control, breaking through in the opening game and then comfortably holding easily.
"I have lost a lot of opportunities through injuries, and I know the years are going quick".
"He's a big favorite against everybody", said Thiem, who reached his first Grand Slam final by ending the out-of-nowhere run of 72nd-ranked Marco Cecchinato of Italy 7-5, 7-6 (10), 6-1.
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The clay courts of Roland Garros host the second Grand Slam of the season.
Del Potro saved one but a backhand error handed Nadal the set and he never took a backward glance after that.
However, he quickly recovered and looked to have the ascendancy, but the Argentine let six break-point chances slip and Nadal pounced with a solitary break in the 10th game which was enough to allow him to seal the set after 54 minutes of high-octane action.
"I know I have to play my best, I have to improve a little bit".
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.
Nadal, by contrast, had not had a sniff on Del Potro's serve until, at 5-4, two swishes of his forehand forced two set points, the second of which he converted when the Argentinian netted.
Thiem used to hike through the Alps in his homeland as part of his training, but the 24-year-old had an even bigger mountain to climb now.
"If I lost that tie-break it would have been a very close match and I didn't want that". "When (Thiem) is dominating the point, he is very risky".
In 2016, Cecchinato was accused of match-fixing and suspended for 18 months, but he appealed, and his punishment was dropped on a technicality.
Cecchinato said: "It was a special tournament for me".
That win, according to three-times former champion Mats Wilander, should be all Thiem is thinking about when he prepares for Sunday's clash.
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