Nadal, who made an unusual number of errors and looked uncomfortable throughout against his 21-year-old opponent, slowly ground him down and fired an ace to clinch his 45th successive victory at the first big claycourt event of the season.
The Spaniard now has a 47-1 win-loss record at the glamorous venue and will next face Frenchman Jo-Wilfried Tsonga, who beat Swiss Stanislas Wawrinka 2-6 6-3 6-4 on the picturesque Centre Court to reach the last four for the first time.
The straight-laced crowd, for once reluctant to support Nadal with Dimitrov threatening to pull off a shock win, stood up to cheer the 34th-ranked Bulgarian as he left the court after a thrilling performance.
Monte Carlo Masters
Murray crushed by Wawrinka, drops back to third
18/04/2013 AT 12:51
Nadal had his share of applause after keeping his bid for a ninth consecutive crown alive by the Mediterranean, where he is coming back from a month-long break after winning three titles including the Indian Wells Masters following seven months out with injury.
"I had a fantastic first set and that's important. That means my level is here," Nadal told a news conference.
"I am losing a bit of intensity at some moments. That's normal after a long time out of competition," added the Spaniard.
In the other half of the draw, Fabio Fognini beat seventh-seed Richard Gasquet 7-6 6-2 to make his first significant breakthrough in a Masters Series event.
The 32nd-ranked Italian, who enjoyed huge support in a stadium located just kilometres away from his country, will next face world number one Novak Djokovic - still recovering from a sprained ankle - or Finland's Jarkko Nieminen.
Tsonga had opened the chilly grey day, with the sun hidden by the clouds and a cool breeze refreshing the sparse crowd, by winning a fierce battle against Wawrinka.
Both players started brightly, with a solid Wawrinka looking on course to repeat the superb third-round performance which knocked out world number two Andy Murray.
However, Tsonga's aggression paid off with the Swiss-based Frenchman making the most of his break chances to overcome his occasional training partner.
Tsonga, who had not reached the last four in a Masters series event since his 2011 home triumph at the Paris Open, saw himself as the underdog ahead of the semi-final with Nadal.
"It's a incredible challenge. If I win, I'll be a hero. If I lose, I'll just go back home like everybody else," he said.
Monte Carlo Masters
Djokovic strolls past Nieminen
19/04/2013 AT 16:58
Monte Carlo Masters
Murray crushed by Wawrinka, drops back to third
18/04/2013 AT 12:51