Rossi's win, his first GP2 success with the Caterham team, came after opting for a medium/medium tyre strategy that thwarted long-time race leader Jolyon Palmer and his Carlin team.
They chose a medium/super-soft strategy, and Rossi's two-lap undercut was enough to propel him in front as Palmer rejoined from the pits.
Rossi won by 2.1 seconds despite a late safety car to retrieve Mitch Evans's stranded car, as Palmer ran the last nine laps on the more fragile tyre and could not take the fight to him at the finish.
Marcus Ericsson passed Leimer for third with three laps to go at Turn 11, but fourth was enough to crown Leimer as champion.
Chief rival Bird stalled at the start, and despite stopping under an early safety car to switch strategy to get the super-soft tyre out of the way, he was unable to rise to the ninth place he required to take the title fight into Sunday, finishing only 10th.
Dani Clos claimed fifth for MP Motorsport, ahead of James Calado, who made a late-race spurt on super-soft tyres despite a broken bargeboard that was starving his engine of power.
Felipe Nasr finished seventh, followed by Johnny Cecotto Jr, who will start Sunday's sprint race from pole.
The early safety car was required for a nasty clash between Jon Lancaster and Tom Dillmann, from which the latter was lucky to escape as Lancaster's front wing was wedged on his crash helmet.
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