One more seeded player was scheduled to be in action later Thursday with No. 2 Nicolas Almagro of Spain up against Federico Delbonis, another Argentine.
Nobody was better on clay on Thursday than the former Wimbledon finalist Nalbandian, who made few errors.
"I played a great match with lots of intensity from the start," Nalbandian said. "Unfortunately, I don't think Juan had his best day. This is a tournament that all Argentines like to play."
Nalbandian will face fellow Argentine Berlocq in Friday's quarterfinals. In other quarterfinal matchups it will be top-seeded David Ferrer of Spain against Chile's Fernando Gonzalez, and No. 4 Kei Nishikori of Japan facing No. 6 Stanislas Wawrinka of Switzerland.
Andreev will face either Almagro or Delbonis.