Montanes will face Frederico Gil, who rallied past Ivan Ljubicic of Croatia 6-7 (2), 7-5, 6-4.
Juan Carlos Ferrero and Victor Hanescu also advanced.
Montanes saved eight of 14 break points against Ventura, while getting seven breaks on his own as both players struggled with their serve.
Ferrero beat Potito Starace of Italy 6-4, 7-6 (1) and will face the fourth-seeded Hanescu, after the Romanian routed Ivan Navarro of Spain 6-1, 6-0.
Ferrero seized a 4-0 lead in the first set before Navarro rallied to 5-4. Ferraro then had two chances to serve out the match at 5-4 and 6-5 in the second but Starace broke each time.
Hanescu needed only 48 minutes for the win, hitting six aces and losing just seven points on his serve.
Petrova beats 14-year-old Keys at Ponte Vedra
