Second-seeded Monaco defeated Taiwanese wild card Jimmy Wang 6-4, 7-5. The Argentine responded to losing his serve three times by breaking Wang five times.
Fourth-seeded Dolgopolov lost to Alejandro Falla of Colombia 6-3, 3-6, 6-3.
"I started really bad and missed a lot of balls," Dolgopolov said. "I came back in the second set, but it was an up and down match."
Falla will face Julien Benneteau of France.
Dutchman Igor Sijsling upset No. 5 Lopez, 7-6 (0), 7-6 (2). They played impeccably on serve with 11 aces each and no break points conceded, but Lopez struggles in both tiebreaks.