The Spanish ace booked his place as he played his 12th semi-final at the Country Club venue, defeating Belgian 10th seed David Goffin 6-3, 6-1.
Nadal will face compatriot and this week’s giant-killer Albert Ramos-Vinolas, who crashed into the final 6-3, 5-7, 6-1 over Frenchman Lucas Pouille.
“It’s tough to believe that I am again in a final here, it is unbelievable news for me,” Nadal said. “It’s another great event.
“I’m be able to start the clay court season playing a final in one of the most important events of the year. Personally, it’s a very special place for me. It’s something that makes me feel very happy. Very excited about it.”
Ramos-Vinolas, 29, who had never been to the final four of an elite Masters 1000 event, began his unexpected run on the clay of the principality by stunning world number one Andy Murray in the third round, and followed by with a dispatch of fifth seed Marin Cilic a day later.
He beat Pouille, not quite a major target as 11th seed, but still a solid opponent.
The contest took two and a quarter hours, with the winner saving seven of nine break points and starting the match with a break of his opponent.
Ramos-Vinolas started the week standing 3-26 lifetime against top 10 opponents. Pouille failed to duplicate the final achieved a year ago by French compatriot Gael Monfils, who lost to Nadal.
“He’s playing better than ever, he’s winning huge matches during the whole week,” Nadal said of his next opponent.
“He’s playing with big confidence, with great spirit of fight, overcoming tough situations in every match, and playing at high level of tennis all the time, no?
“I know that I need to be at my best to keep having chances, to fight for another title here. That’s what I am looking for.”
In the first set, Nadal lost the support of much of the public as the chair umpire missed a ball mark and awarded a point to Nadal midway through, denying the frustrated Goffin a possible 4-2 lead.
With the match momentum now in tatters, Nadal soldiered on while Goffin tried and failed to re-group as he went down in around 90 minutes in his first meeting with Nadal.
“When the umpire goes down and says the ball is good, what I can say? No, the ball is out?” Nadal said.
“This (booing) for me is sometimes sad, especially in a place that I had a lot of success, in a place that I love. Sometimes that happens.”
Nadal is chasing a 70th career title and stands 23-5 in 2017. He leads Ramos-Vinolas 2-0.
This Monte Carlo edition marks the first time since 1998 that three double-digit seeds have figured in the Monte Carlo semis.
Nineteen years ago, number 10 Cedric Pioline defeated number 12 Alberto Berasategui and number 14 Carlos Moya, now Nadal’s coach, beat number seven Richard Krajicek. Moya went on to win the title over Pioline.