Mercedes was fastest in first practice for the 2014 Formula 1 Belgian Grand Prix, with Nico Rosberg outpacing Lewis Hamilton by 0.097s.
As expected, Mercedes-engined cars dominated the top 10, with the 'works' cars trading quickest times in the final half hour.
Rosberg topped the timesheets for the first half of the session with a 1m51.724s lap before Hamilton edged ahead, but Rosberg hit back by going another tenth of a second faster.
Hamilton's race engineer Peter Bonnington informed him that the time came from Rosberg being quicker through the chicane and picking up more mid-corner to exit speed at Pouhon.
Both Mercedes then returned to the circuit in the final minutes but did not improve their times.
Ferrari arrived at Spa with a revised low-downforce aero package and proved to be the fastest non-Mercedes team. Fernando Alonso was especially impressive, third fastest and just two tenths off Rosberg's benchmark.
Kimi Raikkonen, so often a Spa specialist, continued to fight his F14 T but finished fifth fastest, albeit a second slower than his team-mate.
The Ferraris sandwiched the McLaren of fourth-placed Jenson Button, who survived a brief scare when his DRS stuck open. Button was a comfortable half second faster than team-mate Kevin Magnussen.
Red Bull's Daniel Ricciardo was the only other top 10 runner without a Mercedes engine, closing the session in ninth place, behind Magnussen and the two Force Indias of Sergio Perez and Nico Hulkenberg.
Ricciardo's team-mate Sebastian Vettel was two places adrift after an eventful session. He reported gearshift problems and ran off track at Pouhon, then ran just another handful of laps before returning to the garage with a temperature-related exhaust failure.
Valtteri Bottas was the only Williams driver to finish inside the top 10, seven tenths quicker than team-mate Felipe Massa.
While 1.595s separated P1 from P10, the gap from fastest to slowest on the entire grid was a substantial 6.4s, with the usual suspects propping up the order.
Alexander Rossi, who will not now drive in the grand prix, was over a second slower than Marussia team-mate Jules Bianchi.
It only took F1 debutant Andre Lotterer 10 laps at the wheel of the newly upgraded Caterham to overhaul team-mate Marcus Ericsson, who spun at La Source in response.
Pos Driver Team Time Gap Laps 1. Nico Rosberg Mercedes 1m51.577s 25 2. Lewis Hamilton Mercedes 1m51.674s +0.097s 24 3. Fernando Alonso Ferrari 1m51.805s +0.228s 16 4. Jenson Button McLaren-Mercedes 1m52.404s +0.827s 21 5. Kimi Raikkonen Ferrari 1m52.818s +1.241s 17 6. Sergio Perez Force India-Mercedes 1m52.903s +1.326s 24 7. Kevin Magnussen McLaren-Mercedes 1m52.922s +1.345s 23 8. Nico Hulkenberg Force India-Mercedes 1m52.937s +1.360s 22 9. Daniel Ricciardo Red Bull-Renault 1m52.972s +1.395s 19 10. Valtteri Bottas Williams-Mercedes 1m53.172s +1.595s 20 11. Sebastian Vettel Red Bull-Renault 1m53.369s +1.792s 11 12. Daniil Kvyat Toro Rosso-Renault 1m53.594s +2.017s 21 13. Romain Grosjean Lotus-Renault 1m53.597s +2.020s 20 14. Adrian Sutil Sauber-Ferrari 1m53.703s +2.126s 14 15. Felipe Massa Williams-Mercedes 1m53.968s +2.391s 20 16. Jean-Eric Vergne Toro Rosso-Renault 1m54.189s +2.612s 20 17. Giedo van der Garde Sauber-Ferrari 1m54.335s +2.758s 16 18. Pastor Maldonado Lotus-Renault 1m55.336s +3.759s 21 19. Jules Bianchi Marussia-Ferrari 1m55.782s +4.205s 19 20. Alexander Rossi Marussia-Ferrari 1m57.232s +5.655s 20 21. Andre Lotterer Caterham-Renault 1m57.886s +6.309s 24 22. Marcus Ericsson Caterham-Renault 1m57.977s +6.400s 24