He profited from poor luck for title rival Timo Glock who narrowly missed out on a podium finish after a stirring drive.
Giorgio Pantano finished second ahead of Adam Carroll who scraped across the line ahead of Glock.
Luca Filippi made a poor start and Glock fell behind team mate Andreas Zuber at the getaway. This became a problem for Glock when an early safety car period sent the field diving into the pits for fresh tyres.
Glock was among a handful of drivers that stayed out, not wishing to lose time stuck behind their team mates. He took over the lead of the race with a two-car buffer of Alexandre Negrao and Roldan Rodriguez – both yet to stop – between him and effective leader Zuber.
Glock set a blistering pace as he fought to extend his lead as far as possible before the inevitable pit stop.
Zuber effectively held the lead of the race behind Glock and Rodriguez. But with di Grassi not far behind Zuber decided to race Rodriguez – and paid for it.
Trying to pass Rodriguez on lap 22 he ran onto the grass on the run up to the hairpin and flew off the circuit, eventually stopping in a bed of gravel.
It was another low for his iSport team as it left di Grassi first among the drivers who had pitted. He actually held fourth ahead of Carroll, Pantano, Sebastien Buemi (who subsequently retired), Borja Garcia, Kazuki Nakajima and Karun Chandhok.
On lap 25 Negrao pitted and now di Grassi caught second-placed Rodriguez – which meant di Grassi faced the same dilemma Zuber had. He made a few circumspect moves to pass but thought better of it, and sweated it out while Carroll and Pantano caught him. Rodriguez ducked into the pits at the end of the lap.
On the next tour Glock was in which promoted di Grassi, Carroll and Pantano into the podium places. Amazingly Glock resumed behind them in fourth ahead of Garcia.
Glock was nine seconds behind the leaders but even with four laps remaining he looked in with a shout of catching them. Carroll was visibly struggling in the braking zones and Pantano passed him on lap 30. Carroll smartly reversed the move but Pantano made it stick on the following lap.
Glock was 1.1s behind Carroll as the final lap behind and got alongside the FMS racer for second at the final complex. But Carroll clung on to take third.
Di Grassis’s win puts him in the lead of the championship. Pantano followed him home second with Carroll third ahead of Glock. The German came within a whisker of taking the point for fastest lap of Nicolas Lapierre, but failed, the Frenchman having stopped for new tyres on three occasions to set his time.
Garcia finished fifth ahead of Nakajima and Negrao, and eighth-placed Chandhok will start tomorrow’s sprint race from pole position.
Photo: GP2 Media Service / Andrew Ferraro
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