Fourth-seeded Robredo couldn't even reach a break point against Ramos, who won 6-3, 6-4 to reach the quarter-finals.
Bulgaria's Grigor Dimitrov rallied to beat Italy's Filippo Volandri 2-6, 6-1, 6-4. Dimitrov also recovered from a set down in his first-round win against Sweden's Elias Ymer.
Argentines Juan Monaco and Carlos Berlocq earned slots in the quarter-finals after beating Henri Laaksonen and Blaz Kavcic, respectively.
Top seeds Tomas Berdych and Nicolas Almagro enter the tournament on Thursday.