Waiting for the 11-time French Open champion will be longtime rival Roger Federer, who is making his first trip to Roland Garros since 2015.
"Always," Nadal said, "a big match."
It will be the 39th career meeting between Nadal and Federer and their sixth in Paris. Nadal is 5-0 against Federer at the Grand Slam tournament he has dominated and leads 23-15 head-to-head overall.
"If you to do or achieve something on the clay, inevitably, at some stage, you will go through Rafa, because he's that strong and he will be there," said Federer, at 37 the tournament's oldest semifinalist since 40-year-old Pancho Gonzalez in 1968. "If I would have had a different mindset -- to avoid him -- then I should not have played the clay."
The only trouble that Nadal, the defending champion who has a career record of 91-2 at Roland Garros, ran into against the seventh-seeded Nishikori on Court Philippe Chatrier was a rain delay of just more than an hour during the third set.
"I'm very satisfied," Nadal said during his on-court interview after the match. "It's incredible. Lots of emotions. To be back in the semifinals is something very, very important.''
Nishikori was coming off two five-set victories in the last two rounds.
"He didn't let me play how I wanted to play," Nishikori said, "and especially if I'm not fresh, it's not easy to stay with him."
Next up for Nadal will be Federer, the No. 3 seed who defeated Swiss compatriot and friend Stan Wawrinka 7-6 (4), 4-6, 7-6 (5), 6-4 in a match that last 3 1/2 hours.
Federer kept accumulating break points and then frittering them away against Wawrinka in his quarterfinal match, failing to convert 16 of his initial 17 chances.
Then, after a 75-minute rain delay, Federer got his 18th opportunity to try to break -- and he made it count by smacking a big cross-court forehand to a corner that Wawrinka couldn't handle.
Attacking the net at any opportunity, Federer won the point on 41 of his 60 trips forward. It'll be fascinating to see whether he does that against Nadal, too.
It will be the 13th matchup in a Grand Slam event between the two superstars in the Open era; Nadal leads 9-3 in majors and has a 13-2 career edge on clay, on which they last played against each other in 2013 in Rome.
"I really expect him to play aggressive, change rhythms, come to the net," Nadal said of Federer. "That's my feeling. He's playing well and he has the tennis to make that happen.
"I hope to put him in trouble. If not, I will be in trouble."
Federer missed the French Open in 2016 with a bad back, and he sat out the full clay portion of the schedule the next two years to prepare for grass and hard courts.
"My next opponent is not too bad. He can play on clay, unfortunately," a smiling Federer joked to the crowd at Court Suzanne Lenglen. "What a pleasure, actually. If I decided to come back to play on clay, it was maybe to play Rafa."
The Associated Press contributed to this report.