Nadal, the undisputed king of clay, had lost his first set at Roland Garros since 2015 and was a break down in the second, but he regrouped during a 53-minute delay.
When Cilic's final forehand flew wide, del Potro stood legs wide apart, arms aloft, soaking in the crowd that appreciated the effort not just of the match, but of the resolve.
Del Potro, the US Open victor in 2009, is playing the tournament for only the second time since 2012 after a spate of injuries. Now I'm (in the) semi-finals, which means something great to me. "But at the same time, I think after the first rain delay, the match changed a lot because I played more aggressive with high intensity and the things were more on my side".
In a bit of unplanned irony, play began Thursday shortly after Sweden's Robin Soderling - who profited from a combination of heavy weather and self-conviction in 2009 to hand Nadal his first-ever defeat at Roland Garros - responded to numerous inquiries by holding a rare press conference himself.
The world number one was a set and a break down on Wednesday before he regrouped and blazed through the last two sets on Thursday in a last-eight showdown which was played over two days due to rain.
Top-ranked Simona Halep has leveled at one set-all in her French Open quarterfinal against Angelique Kerber of Germany.
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Rafa Nadal found his form when his French Open clash with Diego Schwartzman restarted on Thursday and won 12 of the first 13 points on his way to booking a place in the semi finals with a 4-6, 6-3, 6-2, 6-2 victory.
Earlier this year, del Potro put his relative lack of success on clay down to the fact that he "doesn't like running".
Nadal's streak of 37 consecutive set wins at Roland Garros ended Wednesday, but with victory the 32-year-old became only the third player in the Open era (since 1968) to reach 11 semifinals in the same grand slam.
The fourth set went hurriedly with serve until 5-5, when from out of nowhere Del Potro fashioned three break points.
Nadal is 10-0 so far in Roland Garros semi-finals and he will try to extend his ideal record today against Juan Martin del Potro, setting his eyes on the 11th Roland Garros final, not losing a single final so far between 2005-2017. "It has been a long time without good feelings". I had surgery three times on my wrist and I was close to quitting this sport. I don't have any words.