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 has yet to prove that he can stay with Nadal in a best-of-five-set match, losing in the second round of the French Open in 2014, 6-2, 6-2, 6-3, and again in the French Open semifinals past year by the lopsided and deflating score of 6-3, 6-4, 6-0, after plenty of prematch buildup.
The 24-year-old Thiem, the youngest man since Nadal to reach the final in Paris, ended the contest on his first match point with a forehand victor. But I know how to play against him. They came from non-ATP level matches, which is where the vast majority of tennis' corruption takes place, but corrupt behavior at the lower levels becomes a problem when those players graduate to the big time.
In the quarter-finals, he even dropped his first set in Paris since 2015 to Diego Schwartzman before sprinting to a four-set win. "I just enjoy the fact that I am here again".
'For sure I can take some things from that, ' he said.
"He's an wonderful player", Nadal said of Thiem. "It's going to be a tough one but I will fight all the way". Madrid is at altitude and although Nadal relishes the chance to play in front of Spanish fans, Madrid is a tricky place for him as the balls do fly and he does not have the same element of control he gets in Rome and Paris, which tend to be closer in conditions.
"That's why he's the No. 1 and beating all the guys", added Del Potro after his 10th defeat in 15 meetings with the 32-year-old Spaniard. He missed the tournament every year from 2013-16 because of injuries, including three operations on his left wrist.
In Madrid, Thiem said the key was to be positive against Nadal.
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"I know I have to play my best if I want to have chances", said Nadal, who needs to win Sunday to hold onto his No. 1 ranking. "He beat me in Madrid, he has big power so I have to be at my best and improve", said Nadal.
Nadal and Thiem have met nine times - all of them on clay.
The Spaniard has been beaten just twice on clay in the past two seasons and both defeats have come against Thiem. His head-to-head advantage over Thiem only stands at 6-to-3, and considering the Austrian is only 24-years-old, it's no great surprise that Nadal had the better of their early encounters.
In greenback terms that comes out at $2.6m, making it the smallest winner's prize of the year's four Grand Slam events and almost $1m less than the richest tournament the US Open which paid its 2017 winners - Nadal and Sloane Stephens - $3.7m each.
Romania's Simona Halep will first play American Sloane Stephens in the women's singles final tomorrow.
Cecchinato, who was banned for match-fixing in 2016, had never won a match in the French Open prior to this year's edition. He looks like he is nearly back to his best this season having picked up two titles, which includes winning his first Masters Series title with a three set win over Federer at Indian Wells.
"It was not a very nice feeling", Thiem said.
A fourth set point for Thiem was erased with - what else? - a drop shot by Cecchinato.
The battle wore on and on, before, on the Austrian's fifth set point, Cecchinato went long to fall two sets behind.