The 18-year-old, bidding to become the first player to win the claycourt grand slam at the first attempt since Mats Wilander in 1982, saved a set point in the first set before stamping his authority on the match.
Ferrer, the last Spaniard to beat Nadal on clay, needed treatment on his back in the second set. His younger opponent showed no mercy as he unleashed his full repertoire of shots, clinching victory in two hours eight minutes.
Fourth seed Nadal has now won 22 consecutive matches, all on clay, including Masters Series titles in Monte Carlo and Rome.