Lopez edged past Sergiy Stakhovsky 6-4, 7-6 (7-4). Lopez had six aces and no double faults while winning three-quarters of the points on serve. He saved the only break point Stakhovsky managed.
Fourth-seeded Philipp Kohlschreiber defeated Ernests Gulbis 7-5, 6-3 with one break in each set. Kohlschreiber won 80 percent of the points on serve, losing 11 points in 11 service games.
No. 5-seeded Bernard Tomic, despite losing a second-set tiebreaker, advanced by beating Potito Starace 6-1, 6-7 (4-7), 6-3. He had a pair of breaks in the first and third sets.
Marinko Matosevic took a 6-1, 6-7 (2-7), 6-2 win over Robert Farah in a battle of qualifiers. Matosevic won all but one of the six break points he faced.
Lopez will go against Tomic and Kohlschreiber meets Matosevic in two of Friday's quarterfinals. No. 3-seeded Marin Cilic plays sixth-seeded Mikhail Youzhny and eighth-seeded Marcos Baghdatis squares off against Tommy Haas on the other matches Friday.