Zverev posted a 7-6 (7-5), 6-2 win over Horacio Zeballos. He claimed two-thirds of the points on serve and allowed Zeballos just one break opportunity. That was in the first set and, while Zverev lost it, he also had a break of his own and then took the tiebreaker.
He had a pair of breaks in the second set to advance.
Viola defeated Somdev Devvarman, another qualifier, 6-3, 6-3 with five breaks and won more than half (36-of-71) of the points off Devvarman's serve.
Pablo Cuevas eliminated eighth-seeded Nikolay Davydenko 6-3, 6-4, saving 6-of-7 break points in the process. No. 7-seeded Vasek Pospisil rallied for a 1-6, 6-1, 6-3 win over Victor Hanescu. Pospisil saw just one break point after the first set.
Also Tuesday, Pablo Carreno Busta defeated Alex Bogomolov Jr., 7-6 (7-5), 6-0 and Pablo Andujar advanced when, after Andujar tied the match 3-6, 7-5, Grega Zemlja retired.