Petrov's path to GP2 was unconventional. He skipped competitive karting entirely — karting infrastructure in Vyborg was virtually nonexistent — and instead began motorsport in 1998 through rally sprints and ice races. He spent several seasons in the Russian Lada Cup and Lada Revolution series before turning to European single-seater ladders. By 2006 he had built enough credentials in Euro Formula Renault and Euroseries 3000 to attract a GP2 opportunity.
Petrov made his GP2 debut mid-season in 2006 as a replacement driver for David Price Racing, stepping in for Olivier Pla after Pla lost his Direxiv sponsorship following the German round. The stint was brief but gave Petrov his first exposure to the series at circuits such as Istanbul, Monza, and Spa.
For 2007, Petrov joined Campos Grand Prix alongside Giorgio Pantano. His first full GP2 season produced five points-scoring finishes from 21 starts and his maiden series victory at the Valencia Street Circuit, sending him to 13th in the final standings.
In parallel, Petrov competed in the inaugural GP2 Asia Series in early 2008, finishing third overall behind champion Romain Grosjean and Sebastien Buemi. His win at Sepang International Circuit was a highlight of that winter campaign. Remaining with Campos for the 2008 GP2 Series proper, he ended the season seventh, adding another victory at the Valencia Street Circuit. A fifth-place finish in the 2008–09 GP2 Asia Series — including a sprint-race win at Sepang — kept his momentum heading into 2009.
By 2009, Campos had been rebranded as Barwa Addax. The season was dominated by Nico Hulkenberg, who was close to untouchable throughout, but Petrov mounted the strongest challenge. He took two victories — at Istanbul Park and at the Valencia Street Circuit — and accumulated enough consistent points to finish as championship runner-up. The result, achieved against a highly competitive field that included many future Formula One regulars, established Petrov as a credible F1 prospect.
Petrov's GP2 career demonstrated an ability to extract results from mid-budget teams without the lavish factory backing enjoyed by many rivals. The 2009 runner-up finish was the decisive result that persuaded Renault to sign him as a race driver for 2010, making him the first Russian to compete in the Formula One World Championship. His best-known GP2 trait — precise tire management combined with a willingness to race hard on street circuits such as Valencia — would characterize his F1 style as well. The GP2 period is also notable for overlapping with a rich generation of future champions: Grosjean, Buemi, Hulkenberg, and later compatriots all shared grids with Petrov during these seasons, underlining how competitive the feeder series had become by the late 2000s.