The Jack & Jones Aprilia rider set a best lap time 0.254secs ahead of American Stevie Bonsey, whose second place marks the American's best ever grand prix grid position.
Corsi's team-mate Nicolas Terol, second at
Jerez, will start from third position with Britain's Danny Webb - Bonsey's Degraaf team-mate - completing the front row.
Provisional pole sitter Mike di Meglio speared into the back of a slow moving Ivan Maestro on entry to the chicane, with five minutes to go, and will start eighth on the grid.
Final qualifying - Estoril:
1. Corsi (Aprilia)
2. Bonsey (Aprilia) +0.254secs
3. Terol (Aprilia) +0.255secs
4. Webb (Aprilia) +0.415secs
5. Gadea (Aprilia) +0.441secs
6. Olive (Derbi) +0.487secs
7. Bradl (Aprilia) +0.659secs
8. di Meglio (Derbi) +0.777secs
9. Talmacsi (Aprilia) +0.791secs
10. Redding (Aprilia) +0.962secs
13. Smith (Aprilia) +1.156secs
