However, while Lanzi was able to pull a gap over Sykes straight away, as soon as the Kawasaki man was disposed of second and then third by Checa and the recovering Haslam, it became apparent that it would be a three-way tie for the win.
Checa duly upped the pressure as he reeled in Lanzi, the Althea rider tailing his Ducati counterpart until making a single, successful move for the lead on lap 16. Haslam swiftly followed through to reduce the lead to a two-way battle.
Haslam held station behind Checa, but all signs pointed towards a determined effort by the Suzuki rider on the very final lap, the Briton hinting at his intentions by pulling alongside his rival into Tamburello.
Haslam launched his second attack at the penultimate Rivazza bend, but while he had the inside advantage, he couldn't get his bike stopped in time, Haslam forced to run off the circuit and onto the gravel trap.
Checa simply lifted off to cruise across the line for his second victory of the season, ahead of a delighted Lanzi – who scored his best WSBK result since winning for RG Ducati at Valencia in 2008 -, while Haga completed a Ducati 1-2-3 after a quiet ride up the order.
Haslam recovered to fifth – behind Smrz, who posted his best finish yet on the B&G Aprilia -, a particularly galling result for the outside title contender given arch-rival Max Biaggi endured his toughest race of the season.
Hoping to wrap up the title on home soil, Biaggi tumbled down the order with a mistake at Villenauve on lap three, the Aprilia man lucky to stay upright after a high-speed trip across the gravel trap. A second off-track excursion – caused when he got caught on the outside of Scassa – necessitated a fight back from 15th, Biaggi only able to recover to 11th. Nonetheless, his lead of 52 points over Haslam means he remains on course to win the title in race two.
Outside the top five, Sykes settled for sixth after fading during the latter stages, well ahead of Michel Fabrizio, Shane Byrne and Sylvain Guintoli. Cal Crutchlow completed the top ten as one of several riders to go off the track at some stage.
Behind Biaggi in 11th, Ruben Xaus was 12th, while Scassa, Max Neukirchner and Troy Corser – who went off the road at Tosa from fourth on lap two – completed the points in 15th.
James Toseland, meanwhile, retired with a mechanical problem.