"It was a pain in the arse these three, four weeks, seriously", said Thiem after beating Italy's Marco Cecchinato 7-5, 7-6 (12/10), 6-1 on Friday to reach his first Grand Slam final.
The second set was also dominated by serve until Cilic fired himself up to break at 4-4 - a furious del Potro throwing his racket at the changeover.
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.
"Well, it's tough to speak", the 29-year-old, who has risen back to sixth in the rankings after battling back from three left wrist surgeries, said on court as fans sang "Delpo, Delpo".
On Sunday, Nadal will face No. 7 seed Dominic Thiem, a 24-year-old Austrian who reached his first Grand Slam final by eliminating unseeded Marco Cecchinato in straight sets.
Over the past two seasons, Nadal is 49-2 on red clay, with both losses coming against Thiem: in the quarterfinals at Rome in May 2017, and the quarterfinals at Madrid in May 2018.
A superb backhand crosscourt pass helped Nadal to make the crucial break and with it wrap up the opening set in 54 minutes, but what followed was a procession as Del Potro struggled to contain his frustrations at missing crucial chances to establish a lead.
Kathleen Wynne speaks following the Liberals’ crushing election loss
He is not running for re-election in 2018, making his seat one of the most competitive this time around. The results of the student election will not be released until after the polls close on Thursday.
Del Potro dipped at the start of the third and Cilic had points for a double break.
Cecchinato's bid to become the spoiler's spoiler ended, for all intents and purposes, when he was unable to convert any of three set points in an entertaining second-set tiebreaker that was the middle chapter of Thiem's 7-5, 7-6 (10), 6-1 win.
In 2016, Cecchinato was accused of match-fixing and suspended for 18 months, but he appealed, and his punishment was dropped on a technicality.
Despite rain delaying his passage through to the final, Rafa Nadal has continued his incredible Roland Garros record by reaching yet another showpiece in France. "But I think we always compete well and play good matches".
With that being said, Thiem is on the upswing of his career.
"It was not a very nice feeling", Thiem told the Court Philippe Chatrier crowd afterward.
Del Potro edged the early exchanges even though the scoreboard showed parity, testing Nadal on his service games and earning six break points as a result. "But in terms of play, what changed is how I played and how I managed the match". Thiem got to 10-all with a drop shot of his own.
"That was definitely the key to the match", Cecchinato said.