Del Potro is certainly a worthy opponent and is playing some of his most consistent tennis of his career, but you kind of keep betting Nadal in this tournament until he loses.
Del Potro has won six of his last seven matches on clay court.
Nadal fought off four break points in the decisive eighth game of the third set and added four more break points saved on his way to victory in the fourth set.
He added: "It was a very hard match against a good friend and good player".
The 37-set run was Nadal's longest at the French Open.
Del Potro's win over Cilic on Thursday was an emotional one for the sixth-ranked 29-year-old, who will return to the top four of the ATP rankings for the first time since February 2014 on Monday.
By Thursday normal service had resumed, the Spaniard reeling off the two points he needed to level the match, and then polishing off two more sets to crush this particular French revolution and secure his place in the semi-finals.
Muguruza crucially converted only two of her eight break-point chances and was lacklustre with her second serve at times.
The world No. 1 had found himself in a spot of bother on Wednesday evening, having been broken on five occasions for just the ninth time in 86 matches at Roland Garros, but after initially regaining his footing following a brief rain delay, he was back at his best after an overnight stoppage.
"I am doing well", he said after edging out third seed Cilic 7-6 (7/5), 5-7, 6-3, 7-5. (Smiling). I made too many questions to myself.
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Top-ranked Nadal enters Wednesday's match bidding to reach his 11th semifinal at Roland Garros.
Del Potro said resuming a match at 5-5 in a tiebreak with his serve to come had been a unusual situation but his aim was true and he moved 6-5 ahead before his cagey backhand slice induced a poor forehand error by Cilic. I had surgery three times on my wrist and I was close to quitting this sport.
"It has been a long time without a good feeling in my body".
Del Potro has won only five of his 14 previous matches against Nadal. This really changed totally how I was looking at the match and how my opponent was looking at the match.
Twice the runner-up in Paris, Halep has qualified for the final by defeating 2016 champion Garbine Muguruza 6-1, 6-4.
"I played again one of my best matches on clay", Halep said after coming back from a break down in the second set.
In womens' play, Sloane Stephens and Madison Keys were to play for a spot in the French Open final.
"It's tough to speak now", the 2009 U.S. Open champion said.
Two women who have won the title at Roland Garros, Maria Sharapova and Garbine Muguruza, play each other in the quarterfinals. I am so proud to be playing tennis.