Monaco, the 2007 champion in Buenos Aires, made fewer errors in the second set and took a 3-0 lead in the all-Argentine match. Nalbandian broke twice to take a 5-4 lead but could not finish it off.
In the final, Monaco will face Tommy Robredo of Spain, who beat Jose Acasuso 6-4, 7-6 (3). Acasuso, who received a warning for ball abuse, led 4-1 in the first set but Robredo fought back to win five straight games.
Robredo won his eighth career title last week in Brazil.
The tournament was played through gusts of wind that affected the flight of the ball.
Nalbandian, Robredo into Buenos Aires semis
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