Nyck de Vries held a three-tenths of a second ap over title rival Nicholas Latifi at the end of the 45-minute practice session at the Circuit Spa-Francorchamps.
Latifi, fresh from his Formula 1 outing with Williams, set 11 laps to de Vries' 12, with the latter laying down a 1:59.509 in his ART.
The session ended under a red flag as Ralph Boschung stopped on the circuit with four minutes to go, causing race control to suspend the session and instruct the cars to return to the pit lane.
Boschung's teammate Giuliano Alesi also had trouble as he spun before Turn 11, but managed to avoid hitting the barrier.
Mick Schumacher also ended the session earlier as he found himself facing the wrong way on the exit of Turn 16, stalling the car in doing so.
Louis Deletraz was third while the second DAMS car of Sergio Sette Camara was fourth, ahead of Jack Aitken and Luca Ghiotto.
Despite his session-ending spin, Schumacher was ninth ahead of the Trident pair of Boshung and Alesi, while Nobuharu Matsushita rounded out the top ten.
15:10 - 17:10