In the two Spaniards' fourth meeting at the clay-court major, Nadal maintained his perfect record against Ferrer at Roland Garros, with the world number one claiming a forehand focus in the second set turned the quarter-final around.
Nadal triumphed 4-6 6-4 6-0 6-1 in a rain-delayed match to set up a semi-final clash with seventh seed Andy Murray but admitted he had struggled in the opening set.
"The first set was mentally tough," Nadal said.
"I decided to go more inside to return in the second set, and I decided to play many more times with my forehand.
"When I did, I think the dynamic of the match changed."
Ferrer had overcome Nadal in three of their four matches since he had lost to his fellow Spaniard in last year's French Open final but was left frustrated by a string of errors in the third set, which handed momentum to his opponent.
"Today I was not good enough for this match," the 32-year-old said.
"I lost my focus. I was too slow, and I think I didn't play the game of a top-10 (player).
"This is why I'm sad. It's my attitude, my behaviour on the court."
Nadal will switch his focus to Murray now; a player he has defeated 14 times in 19 ATP World Tour meetings.
While Murray, who defeated Gael Monfils in five sets to reach the semi-finals, has never qualified for the final at Roland Garros, Nadal maintained the British seventh seed will be a formidable opponent.
"He can play very well on all the surfaces," the top seed said.
"It's nothing new that he plays very well on clay.
"It's not the first time he's in semi-finals of Roland Garros...he's a candidate to win (the tournament)."