The French Open champion extended his winning streak on clay to 43 matches, three behind second-placed Bjorn Borg in the Open Era list, to set up a third-round match with Spanish qualifier Ivan Navarro-Pastor.
Nadal said he had been wary of a let-down after his victory over world number one Roger Federer in the final of the Monte Carlo Masters Series event on Sunday.
"I know from experience that the first two rounds are very difficult," Nadal told reporters.
"I know that if I can win the first two rounds then it gets a bit easier after that."
Top seed Nadal broke serve in the second game of the match but immediately dropped his own as Lopez stayed in touch early on thanks to some big serving.
Two double faults helped Nadal break to win the opening set and he broke again in the sixth and eighth games of the second to clinch victory.
Second seed Nikolay Davydenko's title hopes were blown away in a 6-7, 7-5, 6-4 defeat to Croat Ivo Karlovic.
Karlovic hammered 19 aces, including eight in the final set, to set up a third-round clash with Russian Evgeny Korolev, who beat Spaniard Fernando Vicente 6-3, 6-0.
Davydenko, a semi-finalist at the French Open last year, could not cope with the Karlovic serve, failing to convert any of his six break points.
Former French Open champion Juan Carlos Ferrero outslugged fellow Spaniard Alberto Martin 6-4, 7-6 to reach the last 16.
The 10th seed, runner-up to Nadal last year, was a break down at 4-3 in the first set but took three games in succession to move ahead.
Ferrero, the world number 28, led 3-0 in the second but Martin forced a tiebreak, only for his compatriot to romp through 7-1.
Ferrero will next meet another former French Open champion from Spain, Albert Costa, who postponed his intended retirement for another day at least with a shock 6-3, 7-5 victory over eighth seed Dominik Hrbaty of Slovakia.
Sixth seed Tommy Robredo, champion in Barcelona two years ago, cruised past Italian Potito Starace 6-4 6-0 while ninth seed Tomas Berdych lost 7-6, 6-3 to unseeded Spaniard Albert Portas.
Finn Jarkko Nieminen and Frenchman Paul-Henri Mathieu both made the last 16.
