The DAMS driver converted pole position into the lead from the start and simply drove away from the field.
When his team-mate Felipe Nasr was taken out by the Addax entry of Johnny Cecotto Jr, the safety car was deployed and Valsecchi's 12-second lead was eliminated. But the Italian made a fantastic restart and set about building up another huge lead with a series of laps his rivals simply could not match. He duly claimed his fourth GP2 win.
It was a thrilling battle for second, which was eventually won by current points leader Luis Razia. The Arden driver had to work hard for the spot, putting a great move on the Lotus entry of James Calado along the way.
Lotus had vaulted Calado up the order with an early stop, but the tactic failed to pay as for the final half-dozen laps he came under increasing pressure from the squabbling pack behind him. Birthday-boy Max Chilton (Carlin) nipped by to claim third, before Calado's team-mate Esteban Gutierrez breezed past.
Chilton and Gutierrez fought all the way to the finish, with Gutierrez taking the place on the final lap with a great move. Calado fended off Rapax driver Tom Dillmann for fourth, the pair shadowed by the Racing Engineering car of Fabio Leimer, who was set for a potential podium finish before being hobbled by a slow pitstop.
There was a big gap from the top seven back to eighth, which was taken by Coloni's Fabio Onidi, who will start Sunday's sprimt race from pole.
Rio Haryanto took ninth in the second Carlin car for his first GP2 points, while series returnee Brendon Hartley grabbed the final point for Ocean in 10th.