Mercedes driver Nico Rosberg set a dominant pace in the opening practice session ahead of the Belgian GP as the cars returned to the track following the summer break.
Rosberg clocked a 1:48.348 to head his team-mate Lewis Hamilton, who will carry at least a 15-place penalty into the weekend, by over seven tenths of a second despite testing the latest version of the 'halo' cockpit protection device.
Pirelli also used the session to test some prototype tyres for the 2017 season, with a number of drivers taking to the track on the unmarked rubber, designed to withstand "multiple impacts on kerbs or external bodies".
Spa specialist Kimi Raikkonen was third fastest behind the Silver Arrows, eight tenths shy of Rosberg's benchmark, while Sergio Perez enjoyed a strong start to the weekend in fourth and Sebastian Vettel took fifth.
Daniel Ricciardo, who also trialed the halo, was 1.4 seconds off the pace in sixth, marginally quicker than Red Bull team-mate Max Verstappen, while Nico Hulkenberg finished the session eighth.
Williams' Valtteri Bottas and Haas driver Romain Grosjean, who slid into the gravel at T14 late on in proceedings, completed the top ten.
Esteban Gutierrez was 11th, with Felipe Massa 12th and Marcus Ericsson a solid P13 as Sauber tested their long-awaited upgrades, ahead of Carlos Sainz, another to run with the controversial 'halo'.
Manor debutant Esteban Ocon was an impressive 16th fastest as he got to grips with the MRT05, and the only notable mishap came midway through the session as he carried too much speed into his pit-box and slid way past his marks. Nevertheless, the young Frenchman wound up a full second quicker than team-mate and fellow Mercedes prodigy Pascal Wehrlein.
Daniil Kvyat was 17th for Toro Rosso, with Jenson Button down in 18th ahead of Wehrlein, while the two Renaults propped up the order as Jolyon Palmer's morning was cut short by a suspected power unit issue.
Fernando Alonso did not manage to get a time on the board, sidelined for the entirety of the session by a water leak.