Thiem's 35 match-wins this year are the most of any man on the circuit.
There was cause for concern, also, when Nadal quit with a leg injury in the fifth set of his quarter-final against Marin Cilic at the Australian Open this year, heightened when he then withdrew from hard-court tournaments in Acapulco, Indian Wells and Miami - but he has blossomed on the red dirt of Europe.
The 16-year-old McNally, who is unseeded, beat Iga Swiatek of Poland 3-6, 7-6 (6), 6-4.
A fourth set point for Thiem was erased with - what else? - a drop shot by Cecchinato.
"The big key was the second set", he said.
By the absurd standards Nadal has set here, even facing those break points amount to a lapse, as does the opening set he dropped to the terrier-like Diego Schwartzman in the quarterfinals. The No. 1 men's tennis player in the world cruised on Friday - looking vintage in domination -, who entered the French Open semis as No. 6 player in the ATP World Tour rankings.
"He is taking a while to warm up into matches and certainly the (Juan Martin) Del Potro match would be further evidence". Nadal and Thiem have met nine times - all of them on clay.
Once Nadal stole the opener with a break in the 10th game, it was one-way traffic as he left Del Potro trailing in a cloud of Parisian clay dust to set up a final against Dominic Thiem. Thiem withstood a barrage of break points to hold in a marathon sixth game, which he thought he had won at 40-30 when he left a wide ball, only for the umpire to overrule the line judge. Thiem has won 10 straight matches on clay court.
Top-ranked Nadal enters Wednesday's match bidding to reach his 11th semifinal at Roland Garros.
The world No. 8 made history of his own Friday, becoming the second Austrian male to advance to a grand slam final after the hard-nosed 1995 French Open champion, Thomas Muster.
"I was a bit lucky", said Nadal, who is 10-0 in French Open semifinals and 10-0 in finals at the tournament.
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Nadal only dropped one point in the opening two games, going a double break up for a 4-1 lead.
The first set had taken 57 minutes, in stiflingly humid conditions, but there was to be no let-up for the underdog in the second.
"I lost three times until now and no one died", Halep said, "so it will be OK". "Being in a final here is something I should rejoice about and be happy about", he said.
"It's the same old refrain with Rafael Nadal but there's two things I keep on hearing in analysis about beating him". However, after the game, Nadal revealed that he was struggling against Del Potro in the first set.
Watch the final of the French Open LIVE on FOX SPORTS. In fact, plenty would claim him top-three at this point.
Garbine Muguruza overwhelmed Maria Sharapova 6-2, 6-1 in a matchup of past French Open champions to reach the semifinals.
Nadal saved another three break points before holding with a sublime victor, then unleashed a couple of magnificent shots and Del Potro netted a backhand to lose a set which he had the better of.
Although Nadal stormed back against Schwartzman, Schwartzman did dent his flawless record in sets for this tournament.
Schwartzman had a point to lead 4-2 but a forehand error allowed Nadal off the hook.
Simona Halep celebrates her victory against Garbine Muguruza.
"Could he beat Nadal in the final, probably not, but Nadal is not guaranteed to get to the final".