Victory would give Nadal an 11th title in Paris and equal Margaret Court's all-time record for wins at the same Slam event - in her case, the Australian Open from 1960 and 1973.
This time, Thiem instead faced Cecchinato, a 25-year-old from Sicily who never had won a Grand Slam match until this tournament and was the lowest-ranked men's semifinalist at the clay-court major in 19 years.
Nevertheless, the 24-year-old can look back on two victories over Nadal on his favourite surface in the last 13 months, including one in Madrid just last month. For me it's incredible. "I know I have to play my best if I want to have chances".
"My feeling is on Sunday I have a very hard match against a player that is playing great", said Nadal, who has won six times out of nine matches against Thiem, including in the semifinals of the previous French Open. "If it's going to be him on Sunday, I will try everything that my plan also going to work out a little bit here and not only in Madrid or in Rome", Thiem said.
Robin Soderling, who famously inflicted Nadal's first defeat at the French Open in 2009, insists Thiem has to take the initiative from the start.
"He is a nightmare opponent for Nadal", Wilander said about Del Potro, as quoted on Tennis World USA.
Thiem "plays very heavy topspin, very aggressive, goes for the shots, [has] fitness, condition", Moya said. "I hope that I can win", Nadal commented on Thiem. But Nadal saved all six and broke in game 10 to take a one-set lead.
Only two men have stopped Rafael Nadal in the past and they have merely proven to be bumps in the road as the King of Clay lifted aloft the French Open crown jewels on no fewer than 10 occasions. In the fourth game Friday, del Potro clutched at his left hip after being wrong-footed by one shot from Nadal and was visited by a doctor at the next changeover. "I just enjoy the fact that I am here again".
He does see new maturity in Thiem, more controlled aggression on critical points and more skill on the attack. "I couldn't make it", said Del Potro.
"He's a big favorite against anybody", Thiem said of Nadal.
They tried their best to rouse their man in the third set but it was mission impossible as Nadal, clubbing the ball as if his life depended on it, churned out winners.
"He's been a very complex opponent".
Expect Thiem to be the second player to take a set against Nadal.
"If he continues with this desire, he can win many more times, like Federer for Wimbledon, and other major tournaments", said Del Potro whose own Grand Slam collection remains stuck on one - his 2009 US Open breakthrough.
Cecchinato, who was suspended for match-fixing in 2016 before being cleared on a technicality, proved to be no pushover even though he had never won a match at a major before this year's French Open.
The Argentine won the first two points to round off the first set and outlasted his Croatian opposite in lengthy rallies to win in three hours and 50 minutes.
"It was not a very nice feeling", Thiem told the Court Philippe Chatrier crowd afterward.
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.
At long last, Thiem converted his fifth set point when Cecchinato sent a forehand long.