But fast-forward 12 days and the affable Del Potro is preparing to take on 10-time champion Nadal in his first French Open semi-final since 2009.
Nadal and Thiem will face off on Sunday at a 9 a.m. ET to close the French Open.
Croatian Cilic played an terrible game when serving for the set, making four unforced errors, but he was gifted another chance to serve for the set after breaking Del Potro again and at the second time of asking he levelled the match.
"I know I have to play my best, I have to improve a little bit". Trying to beat Nadal at Roland Garros in a best-of-five match is akin to trying to beat '08 Phelps in the individual medley, or 2012 Usain Bolt in the 200-meter dash.you get the picture.
"He portrays someone who believes in himself as much as Federer, more than (Novak) Djokovic and more than (Andy) Murray".
"I think that was my chance of the match". They think, what's he up to?
"He is tall so Rafa's shots don't bounce over his shoulder, he has big weapons, the serve which gives him free points and the forehand which is the biggest in the game and he can hit through Nadal's forehand with it". Both played superbly - and both had chances to end it.
"It's tough to speak now, " the fifth-seeded Argentine player said. It is nice to get the two best clay courters possible in Sunday's final.
"But he's lefty and he can find easily my backhand, and he knows what my backhand is at the moment". I mean, not many players can beat him on clay.
Rafael Nadal lost his first set at the French Open since 2015 but that blip did not prevent the 10-times champion from marching into the semi-finals with a 4-6 6-3 6-2 6-2 victory over Diego Schwartzman in their rain-delayed match on Thursday.
World 72 Cecchinato had knocked out three seeded players to reach the semi-finals, including 12-time major victor Novak Djokovic in the quarter-finals.
Also, while the players had the same amount of unforced errors with 26, Thiem was rewarded for his aggression as he prevailed in the winners battle 37-24.
But first he must step into the unknown.
"This is one of my favourite tournaments and I'm looking forward to Saturday".
"Can he play with pressure?"
Yes, I feel that I faced the match mentally as I had to. Nadal has won 11 straight matches on clay court.
This was del Potro's first semifinal appearance at a Grand Slam since making a comeback after his injury three years ago.
The biggest key was the second-set tiebreak, as Thiem outlasted Cecchinato 12-10 to take a 2-0 lead in sets before steamrolling the unheralded Italian in the third.
In brighter and quicker conditions than those of Wednesday, Nadal's experience told as he came out more aggressively and swept away his 5' 7' opponent, who had made such a gallant run to the last eight.
After nicking the first set with a break in the final game, Nadal cracked 13 winners and allowed Del Potro just 16 points in a second set lasting just half an hour.