The three-time French Open champion's only loss during that stretch - dating back to 2005 - came against Roger Federer in last year's Hamburg Masters final.
"That's not bad,'' Nadal said about his record.
Taking control from the start, Nadal forced Lopez to save three break chances in the third game to hold his serve. Nadal, the three-time defending champion here, instead broke in the fifth game when the 35th-ranked Lopez hit long.
Nadal's backhand was sharp throughout the match, and another break in the second set was enough to set up a quarterfinal match against Juan Ignacio Chela of Argentina, who beat Latvia's Ernests Gulbis 2-6, 6-4, 6-3.
Nadal, who was sporting bandages across his left shoulder and under both knees, had been critical of the scheduling of clay-court tournaments coming into Thursday.
The second-ranked Spaniard next plays at Rome and Hamburg, and then has a one-week break before the French Open.
Stanislas Wawrinka of Switzerland will play Albert Montanes of Spain after upsetting seventh-ranked David Nalbandian of Argentina, 6-3, 6-1. Montanes held on to beat Russia's Mikhail Kukushkin 6-3, 3-6, 6-3.
Earlier, fifth-seeded David Ferrer of Spain rallied to beat Nicolas Lapentti 4-6, 6-4, 6-3.
Ferrer saved three break points in the second set before Lapentti hit long in the 10th game to give him a chance to break for the set. The 83rd-ranked Ecuadorean double-faulted to tie it at 1-1.
Ferrer then broke his opponent in the sixth game of the decider when Lapentti sent a weak backhand into the net. Ferrer saved two break points in the in the final game before clinching the match. He will next play countryman Tommy Robredo, who beat ninth-seeded Guillermeo Canas of Argentina 6-1, 7-5.
Robredo, the last player to win here prior to Nadal, eased through the first set and held a 4-1 lead in the second. But Canas rallied and led 5-4, before Robredo won the last three games, clinching the match when Canas hit into the net.
Denis Gremelmayr beat 15th-seeded Dmitry Tursunov of Russia 6-4, 6-0 to face Nicolas Almagro of Spain. Almagro improved to a 21-3 record on clay this season after beating Croatia's Mario Ancic 7-6 (5), 6-2.
Medina Garrigues reaches Fez quarters
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