A shoulder injury forced Almagro to miss Sydney and the Australian Open.
Of his 12 titles on the ATP Tour, all on clay, six were on the Latin America swing, the most of any player. He's won in Brazil, Argentina and Mexico, and bidding for his first win in Chile.
He will face first-time ATP quarterfinalist Daniel Taro, who beat eighth-seeded Federico Delbonis of Argentina 1-6, 7-6 (5), 7-6 (6). Taro, a Japanese who entered as a qualifier, was born in New York but trains in Valencia, Spain, where he learned the clay-court game.
Unseeded Daniel Gimeno-Traver of Spain beat Hector Zeballos of Argentina 6-2, 6-4, and his opponent in the last eight will be Leonardo Mayer of Argentina, who advanced on Wednesday with a victory over No. 2 Tommy Robredo of Spain.
Rafael Nadal, the world's top clay-court player, skipped this event. On Thursday he pulled out of next week's event in Buenos Aires, saying a stomach virus had kept him from practicing. He also is recovering from a back injury. It is not clear if he will play the following week in Rio de Janeiro.