Fourth became third when Melandri dipped beneath Haslam for third on lap twelve, though it failed to break up the closely fought group with Haslam, Rea, Fabrizio and Laverty, the Italian having also passed the 2010 Supersport runner-up, all in his shadows.
Rea would be the first to crack when he was caught out at Honda hairpin on lap 14, the Ulsterman running off the track and onto the wet grass, dropping him outside of the top ten.
Unable to pull away, Melandri was back in the clutches of Haslam as they entered the final laps, the Briton snatching back the position with four laps to go.
Resisting Melandri's renewed advances on the final lap, Haslam was able to hold onto the podium on his maiden outing with BMW. Notably, the result also marks only the third podium finish for the manufacturer in WSBK.
Denied a top three finish on his WSBK debut, Melandri would also be pushed down to fifth at the death when an untidy exit from the final turn enabled Laverty to eke ahead on the line and take first 'inter-team' blood against his Yamaha counterpart.
Fabrizio followed closely in sixth aboard the sole Suzuki Alstare, eight seconds up on seventh place Jakub Smrz, who gave Effenbert-Liberty Ducati something to smile about in a race that saw Sylvain Guintoli make a mess of his start to drop from third to 18th before then crashing out altogether.
There were also mixed fortunes for Kawasaki, with Tom Sykes making up two positions in the final laps to finish a solid eighth on the new ZX-10R. However, Joan Lascorz – who had been running ahead of Sykes – suffered a fall at Lukey Heights.
Veterans Haga and Corser rounded out the top ten after failing to build on their early race pace, ahead of series returnee Roberto Rolfo, who put in an unnoticed – but nonetheless impressive – performance to finish in 11th position from 21st on the grid for Team Pedercini's best WSBK result in three years.
Rea recovered to a frustrating 12th, ahead of Leon Camier in 13th, the Briton doing a mighty job to even reach the chequered flag having speculated pre-race that he may last only 'three of four laps'.
Ayrton Badovini and wild-card Bryan Staring rounded out the points in 14th and 15th, ahead of a disappointing Ruben Xaus, while James Toseland and Josh Waters would also miss out after tangling on lap four.