The sixth-seeded Argentine beat the unseeded Spaniard in straight sets, 7-6, 6-1 to reach the final eight.
Meanwhile, another Argentine player - defending French Open champion Gaston Gaudio - reached the quarterfinals with a straight-set win over Romanian qualifier Victor Hanescu 6-2, 6-0.
Italian player Filippo Volandri overcame cramps, elbow pain and a knee injury to beat Argentina's Mariano Puerta in three sets 6-7, 7-6, 6-4.
Later Thursday, world number one Roger Federer of Switzerland plays Fernando Gonzalez of Chile. Former French Open champion Juan Carlos Ferrero of Spain faces Australian Open winner Marat Safin of Russia.
The tournament is one of several clay-court events that players are using to prepare for the second major tournament of the year, the French Open, which starts May 23 in Paris.