Zeballos fired eight aces to defeat Nalbandian 6-2, 7-5 and earned a quarterfinal berth against Potito Starace. Zeballos reached the quarterfinals in just three tournaments last year.
Starace advanced with a 6-4, 6-1 decision over Pablo Andujar.
Chela will face Giraldo in the quarterfinals. Nalbandian's exit leaves Thomaz Bellucci as the top seed remaining at No. 3. Bellucci will face Fabio Fognini Friday in the quarterfinals.
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