Cecchinato had never won a grand slam match until the French Open.
Increasingly frustrated at Cecchinato's resistance, Thiem lost his composure completely in the second set.
She improved in the second set, breaking early on only to be pegged back at 4-4 before Halep held a marathon ninth game and broke for a 6-1 6-4 win. Rain meant that the final was completed on Monday which was tough on Djokovic who was a break up in the fourth set when the match was halted for the night.
Play in the men's quarterfinals has been stopped again due to rain at the French Open.
There was a sense of inevitability when Del Potro served at 4-5 and Nadal turned the screws to grab two set points at 15-40.
And the underdog soon faded as Thiem kicked on, racing to two swift breaks in the third and rescuing two break points to serve out the match in style. Thiem has won 10 straight matches on clay court. "However, there is no denying that Nadal, on clay at Roland Garros, on Centre Court, in the final, is tennis' greatest challenge".
Rafael Nadal progressed to his 11th French Open semi-final on Thursday.
Looking ahead to the final and recalling Thiem's victory over Nadal in Madrid, a exhausted del Potro uttered: "Maybe on Sunday (Thiem) can repeat". As del Potro served, he was distracted by a noise from the crowd and double-faulted to lose the game.
Nadal had looked imperious on his way to a 12th quarter-final at Roland Garros, but the diminutive Schwartzman played with aggressive intent to rock the claycourt king.
The injury-prone Argentine had a discussion with Dr. Bernard Montalvan during the changeover, with Nadal leading 3-2 in the first set. "So I need more work for go top 10 or top 20, so these matches was very important for me".
"This is one of my favourite tournaments and I'm looking forward to Saturday". "I'm not the one who has the pressure".
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.
This was the No. 5-seeded del Potro's first semifinal match at Roland Garros since 2009.
Top-ranked Simona Halep will have another shot in the French Open final.
Muguruza, a two-time major champion, entered the semifinals having not lost a set in the tournament.
Otherwise, he stayed steady in a 7-6 (5), 5-7, 6-3, 7-5 win over No. 3 Marin Cilic in a matchup between two past U.S. Open champions. There may be some shocking results over two sets, but three are different.
The degree of Nadal's domination can also be deduced from the fact that he has thrice won the French Open without dropping a set (the most in the Open Era, by a player on a single surface).
The 10-time champion needed a downpour on Wednesday evening to rescue him from a deluge of break points, five of which were converted by tricky Argentinian Diego Schwartzman. Both played superbly - and both had chances to end it. Thiem went ahead 6-3, but wasted three set points there, including a bad volley into the net at 6-4 that left him chewing ruefully on his left index finger.
The record for the clay-court Grand Slam tournament was Bjorn Borg's 41-set winning streak from 1978-81.
"Could he beat Nadal in the final, probably not, but Nadal is not guaranteed to get to the final".