Together with Lorenzo Lanzi
coming down on lap three, as well as Spies and Fabrizio's accident, four of the top ten contenders were already out of the race, while an unscheduled pit-stop for Tom Sykes had also ruled him out.
With Biaggi now comfortably in charge, Ten Kate Honda pair Rea and Carlos Checa, up from tenth on the grid, were left to fight over second position, the Spaniard getting the edge on his younger team-mate on lap five.
Further back, all eyes were focused on Haga too, who was putting in a fine performance as he attempted to capitalise on Spies' unexpected retirement. Rising to eighth position, Haga's hopes of avoiding a strenuous race were ended as he became embroiled in an intense tussle for sixth with Leon Haslam, Makoto Tamada
and Jakub Smrz.
Having made an excellent start from 17th position on the grid, Haslam led the quartet for the majority of the race, resisting the attentions of Haga firstly and then Tamada, the returning Japanese rider producing possibly his most convincing ride on the Kawasaki to date.
As it happens, all three would fall foul of Smrz, the local favourite being cheered on as he picked his way back up the order from a 12th place starting position. He would eventually pass Haslam three laps from the chequered flag to secure an encouraging sixth place finish.
Back at the front, despite Checa's best attempts to reel him in, Biaggi was untroubled as he made sure of keeping the RSV-4 upright through the final turns to claim a marvellous victory for the Italian team. His win also makes him only the fifth different race winner this year, while it also moves him back up in the fight for fourth overall.
Checa finished a lonely second for what remains one of his most convincing performances of the season, while Rea completed a good race for Ten Kate Honda in third, despite having been unable to match his team-mate in terms of race pace.
The battle for fourth came down to an exchange between Corser and Shane Byrne, the Briton stalking his way back into contention before overtaking the former champion on lap 14. Fourth marks his fifth top six result in five races.
Despite slipping back in the closing stages, fifth still represents BMW's best-ever result at World Superbike level, Corser's inspired performance the best indicator yet of the team's potential at this level.
Behind Smrz in sixth, Haslam was a satisfied seventh having endured a troublesome weekend so far, although his Stiggy Honda team-mate John Hopkins didn't fare so well when he crashed out whilst running in tenth position.