Unseeded Horacio Zeballos beat fellow Argentine David Nalbandian 6-2, 7-5 on Thursday to knock the No. 1 seed out of the Movistar Open, the first clay-court tournament of the ATP season.

It was the first match between the two and the loss shocked Nalbandian, who had been off to a good start this season. It's Zeballos' first tournament of the year.

Three other players joined Zeballos on Thursday in the quarterfinals: No. 8 Santiago Giraldo, No. 4 Juan Ignacio Chela and No. 5 Potito Starace.

Giraldo of Colombia reached his third quarterfinals this year when he beat Igor Andreev of Russia 7-6 (7), 4-6, 6-2. After missing four break points in the second set, Giraldo dominated the last set to finish off Andreev in just under 2½ hours.

Fourth-seeded Chela advanced to the quarters in Santiago for only the second time in eight attempts after defeating Brazilian qualifier Caio Zampieri 6-2, 6-3.

Fifth-seeded Starace also progressed, defeating Pablo Andujar of Spain 6-4, 6-1 for a fifth straight time.

The quarterfinal pairings are: Zeballos vs. Starace, Chela vs. Giraldo, Fabio Fognini vs. Thomaz Bellucci and Tommy Robredo vs. Maximo Gonzalez.