Top seed David Nalbandian suffered a shock straight-sets defeat by Peru's Luis Horna in the round-robin stage at the Buenos Aires Open on Tuesday.

Nalbandian's fellow Argentine Agustin Calleri, seeded three, also lost, falling to Spaniard Albert Montanes in his opening match.

Horna, who won the Vina del Mar tournament three weeks ago, stunned Nalbandian 6-4, 6-3 in the opening match in Group One.

"I knew how I had to play and I did it for the whole game," Horna told reporters. "I took all the chances he gave me."

"To beat David in Buenos Aires is an important win."

Calleri went down in two hours 19 minutes to Montanes, who avenged his defeat by the same opponent at Costa do Sauipe six days ago.

Although the round-robin system will give Calleri a second chance, the Argentine is not impressed by the format of the tournament.

"I don't like the round-robin system, I hope it isn't implemented," he said. "I like to get to the quarter-finals by winning games, not on difference of sets."