He remains on course to again meet top-seeded Novak Djokovic in the final, after beating the Serb last year.
"To be in the semifinals is a very good start to the clay season," said Nadal, who is still chasing his first title of the year. "The result against one of the tougher opponents right now on clay is (a) good one."
Although Nadal was disappointed with a "couple (of) mistakes" on his backhand, he felt happy with his overall play.
"I had good control of the ball, when I attacked with the forehand I did well," he said. "My serve was much better today than the rest of the three days. That's very important, too."
Djokovic produced his best tennis of the tournament to dispatch David Nalbandian 6-2, 6-3, breaking the Argentine's serve five times.
He plays No. 6 Fernando Verdasco, who beat Albert Montanes 6-3, 6-7 (4), 6-0 in an all-Spanish match.
Verdasco looked certain to cruise to victory after leading 5-1 in the second set, but Montanes clawed his way back.
Verdasco, however, regained his composure and raced through the deciding set, sealing victory on serve when Montanes hit a forehand wide.
Djokovic took a lighthearted approach at being the only non-Spaniard left in the tournament.
"If I'm representing the rest of the world, then it must be a big responsibility," he said. "It's not a surprise if there are so many Spanish in the quarterfinals or semifinals. They're growing up on this surface."
Djokovic served fluently throughout and was consistent on both first and second serve, and called his performance "one of the best matches I've played in 2010."
His ability to mix it up thwarted Nalbandian's strokeplay and disrupted his rhythm.
"He has one of the cleanest, nicest tennis shots on the tour," Djokovic said. "So I (tried) to move him around, get him off balance. I managed to do that, so he made a lot of unforced errors and made my life easier."
Second-seeded Nadal had dropped only two games in his previous two matches, but this time the ninth-seeded Ferrero _ the Monte Carlo Masters champion in 2002 and '03 _ did at least provide some testing moments for the Spaniard.
Ferrero played the shot of the match with a splendid backhand winner down the line which completely wrong-footed even the agile Nadal as he fought to stay in a keenly contested all-Spanish match between former French Open champions.
"I think I had the chance to win some more games, but I didn't," Ferrero said. "(Nadal's) always there, he's always where he has to be. He plays better in the important moments."
Although Ferrero stayed in most of the rallies, troubling Nadal at times with his sudden changes of pace, he fell short in his shot selection when seeking winners.
"I dominated the points a lot of times, and I couldn't finish," Ferrero said. "Maybe I needed a little bit more power to push and push all the time."
Ferrero saved two match points before Nadal closed out the contest with a strong serve that Ferrero could only return into the net.
Nadal next meets David Ferrer in another all-Spanish match, having beaten Ferrer recently at the Sony Ericsson Open in straight sets.
Nadal leads Ferrer 9-3 since Ferrer won their first meeting, on clay, six years ago in Stuttgart, Germany.
Ferrer earlier beat Philipp Kohlschreiber 7-5, 7-6 (1).