If Nadal's march toward an 11th championship at Roland Garros was going to be stopped on this day, things were going to have to go del Potro's way right then. Injuries have hampered the King of Clay throughout the past year.
On Friday, he was on par with the Austrian seventh seed Dominic Thiem for two sets and even had set points to level the contest at one-set all.
Nadal has also won six of his last eight grand slam final appearances and is 10-0 in French Open final appearances.
The pivotal part of the match was the second-set tiebreaker in which both men 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. Nadal is 49-2 since 2017 when playing on clay.
The toughest task yet is yet to come though, when he will face someone who had reached the final at Roland Garros 10 times before and never lost.
Earlier, the Japanese duo won 6-2, 6-2 against Hao-Ching Chan of Taiwan and Zhaoxuan Yang of China 6-2, 6-2.
While there have been multiple other great single-surface players, the ones listed above have shown both domination and a certain degree of longevity in their careers on these surfaces.
The first set always looked key and Del Potro clinched the opener as Cilic blinked first.
"You can win a set, but not a match against Rafa", said Del Potro keen not to over-egg any hint of weakness in Nadal's game.
"I'm really happy that I won this Grand Slam".
Del Potro, the 2009 U.S. Open champion, has a 1-3 record in Grand Slam semis. The final starts at 6.30 pm IST.
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The first semi-final was between Thiem and surprise semifinalist, Marco Cecchinato, and it was a hard fought match with the first two sets setting the court alight.
Thiem weathered three set points for Cecchinato, at 7-6, 9-8 and 10-9.
Bresnik likes Thiem's current state of mind and form after encouraging him to play at the low-level ATP event in Lyon, France, the week before Roland Garros. Cecchinato was accused of match-fixing and suspended for 18 months in 2016, but he appealed, and his punishment was dropped on a technicality.
Nadal has dropped only one set so far in reaching the final - one more than he conceded in winning the 2017 title.
Thiem then held his serve at love for a 2-0 lead. Perhaps. But he has not won a grand slam since he beat Andy Murray here two years ago.
The boys' final at the French Open will pit top-seeded Sebastien Baez of Argentina against Chun Hsin Tseng from Taiwan.
That put Thiem, a 24-year-old from Austria who is seeded No. 7, in the semifinals at the French Open for the third consecutive year.
Later Friday, Nadal looks to take another step closer to an 11th French Open title and 17th at the majors.
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.
Thiem has been to the semifinals in Paris three years in a row.
Thiem is appearing in the final four at Roland Garros for the third consecutive time.
But while the Swiss has succumbed in three title matches in southwest London, Nadal has never tasted defeat in a final in southwest Paris, as Bresnik - and the rest of the tennis world - knows.