Del Potro missed six break points in the opener, including three at 4-4.
Brett: I don't know if Rafael Nadal would've lost to Diego Schwartzman if there wasn't the delay, but the delay certainly didn't hurt the ten-time Roland Garros champion. For me it's incredible. 'If I want to beat him, I have to play that way like I did in Rome and in Madrid. And I had my chances today, but I couldn't play my best because of him. "But I'm also aware that here it's tougher", said Thiem, the first Austrian in a major final since Thomas Muster won in Paris in 1995.
Del Potro, playing in the semi finals here for the first time since losing to Roger Federer in 2009, was 0-40 ahead on Nadal's serve at 1-1 but could not press home the break and worryingly appeared to hurt his hip at deuce in that game.
Thiem is the only man to defeat 10-time French Open champion Nadal on clay this season.
Both players contributed to an entertaining start, but Thiem quickly took control, breaking through in the opening game and then comfortably holding easily.
Prevailing on Nadal's favourite court in Paris over five sets will be far harder, however.
The 10-time Roland Garros champion was second-best in the first set, but the powerful Del Potro was unable to claim a solitary break as he was thrashed 6-4, 6-1, 6-2 on a glorious Friday in Paris.
Nadal's streak of 37 consecutive set wins at Roland Garros ended Wednesday, but with victory the 32-year-old became only the third player in the Open era (since 1968) to reach 11 semifinals in the same grand slam.
"If I had lost [that set], it would have been a very close match", Thiem said.
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Nadal won the French Open on his debut as a 19-year-old and came into this year's event with a 79-2 win-loss record at Roland Garros.
Nadal will be chasing his 11th singles title here on Sunday, if rain does not push the final to next week.
In 2016, Cecchinato was accused of match-fixing and suspended for 18 months, but he appealed, and his punishment was dropped on a technicality.
Thiem was the aggressor early on and he broke in the first game before holding to love for a 2-0 lead. 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.
It will be Nadal's second Argentine opponent in as many rounds after Del Potro defeated Marin Cilic 7-6 (7-5), 5-7, 6-3, 7-5 in the other quarter-final to book his place in the last four.
The battle wore on and on, before, on the Austrian's fifth set point, Cecchinato went long to fall two sets behind. It was the last ATP match of his second career [returning after a freak roadside accident in Miami in 1989].
People may point to the fact that Thiem recently beat Nadal in his own backyard in Madrid, as it were, but there are few things to remember here.
However, having squandered one set point, he went in for a simple backhand volley victor only to dump it into the net.
"That was not a nice feeling". "It was destructive a little bit", Thiem, who won two claycourt tournaments this season, said.
Cecchinato was understandably deflated - the third set lasted a mere 24 minutes - and Thiem officially booked his spot in the final, to his mother's delight.
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