Nalbandian, the top seed, had a 6-3, 3-0 lead when Ferrero retired. That set up a semifinal meeting with Juan Monaco, who was a 6-2, 6-3 winner over Maximo Gonzalez.
In the other semifinal, third-seeded Tommy Robredo will face Jose Acasuso.
Robredo, winner of last week's tour stop in Brazil, captured his eighth straight match with a 6-4, 7-6 (7-4) decision over qualifier Franco Ferreiro.
Acasuso, who lost to Nalbandian in last year's Buenos Aires final, easily topped Oscar Hernandez 6-2, 6-2.