"It was a very hard first set and Juan Martin had lots of opportunities".
And with Del Potro serving to stay in the set at 4-5, his failure to convert any of his six break points came back to bite him. "But all my semi-finals in Grand Slams were against them".
"It's impossible without hard work and going through tough moments".
"I will keep it for the final and after I will tell you".
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But the Spaniard was impressive on Friday, finishing with 35 winners and 19 unforced errors to Del Potro's analysis of 20 and 32.
"But I believe I will be ready for the final".
There was a sense of inevitability in the air when Del Potro served at 4-5 and sure enough Nadal turned the screws to grab two set points at 15-40.
Del Potro was in the semi-finals for the first time since he recovered from three years of injury problems that made him contemplate quitting the sport.
"Del Potro has achieved great victories this year", said Nadal who dropped his first set at Roland Garros in three years in his quarter-final victory over Diego Schwartzman. Stephens won last year's US Open; Halep is 0-3 in major finals. Having failed to earned a single break point throughout the set, Nadal pounced when it mattered.
Del Potro found the breakthrough at 5-5 in a tight fourth set, when Cilic sent a backhand long to give him a break and he went on to serve out the match.
Del Potro missed another chance at 2-0 in the second set and from that point there was only going to be one victor.
"I'm not the one who has the pressure", he said.
"Rafa served well and he made me run a lot".
Nadal giving up a 6.5 game head start is [1.74] on the Exchange, and I'd expect this to rise by several ticks as the market forms in the run-up to the match, and I'm quite happy to look at this as a more viable proposition. "He deserved to win".
Thiem also defeated his rival in Buenos Aires in 2016, joining Novak Djokovic and Gaston Gaudio as the only men to have beaten Nadal three times each on clay.
Play resumed with the towering fifth and third seeds locked at 5-5 in the first set tiebreak and del Potro needed only two points to pocket the first set on Thursday. "The big key was this set".
"It was not a very nice feeling", Thiem said.
Halep has been beaten in all of her previous Major final appearances, including at Roland Garros past year by Jelena Ostapenko.
World number 72 Cecchinato stunned Roland Garros on Tuesday by following up his surprise wins over top-10 seeds David Goffin and Pablo Carreno Busta with a breathtaking victory against 12-time major champion Djokovic.
But Philippe Chatrier "is his court", a smiling Moya said of Nadal.