Argentine qualifier Federico Delbonis staged the biggest upset of the day at the VTR Open on Thursday, when he upset third seeded and former champion Thomaz Bellucci 6-2, 7-5 to book his spot in the quarter-finals.

Delbonis broke 2010 champion four times out of five and was broken only once in the match. He will next face countryman Carlos Berlocq, who ousted another Argentine qualifier Diego Junqueira 6-1, 6-0.

In another awaited match of the day,  four-time former champion Chilean wild card Fernando Gonzalez lost to Brazil’s Joao Souza 6-3, 7-6(6).

"That's how tennis works," Gonzalez said. "You never feel the same every day. Today I didn't feel comfortable on court and also Souza played very well and took almost all the chances he had."

Souza will next meet second-seeded Argentine Juan Ignacio Chela, who defeated countryman Horacio Zeballos 6-4, 7-6(2).