Nico Rosberg outpaced Lewis Hamilton by seven tenths of a second in Practice One for the Belgian GP as the F1 season resumed following the summer break.
Rosberg's fastest time was set with the Halo device attached to his car.
On the morning in which it was confirmed Hamilton will suffer at least a 15-place grid penalty for exceeding engine allocation limits, Rosberg assumed the position on the timesheet he is expected to have into the weekend.
Both Mercedes drivers ran with fresh engine elements, but Rosberg remains inside the penalty-free limit of five.
Rosberg set a fastest time around the fearsome Spa circuit of 1:48.348, with the lap set while the German had the Halo head-protection device fitted to his Mercedes.
Four drivers - Daniel Ricciardo, Nico Hulkenberg, Carlos Sainz and Rosberg - trialled the Halo as F1's assessments of the device continue ahead of its planned 2018 introduction. The Mercedes was the only car to complete timed runs with it attached.
Four-time Spa victor Kimi Raikkonen lapped within a second of Mercedes on an encouraging morning, with the Ferrari driver third ahead of Force India's Sergio Perez.
Sebastian Vettel, who lost time through a brake change, was fifth with Red Bull duo Ricciardo and Max Verstappen sixth and seventh respectively.
Fernando Alonso was sidelined by a water leak and completed only three laps as McLaren worked to change his Honda power unit, which had only been newly installed for P1 following an upgrade by the Japanese suppliers.
Following his arrival at Manor over the summer break in place of Rio Haryanto, Esteban Ocon impressed on his debut for the team and promptly outpaced fellow Mercedes protégé Pascal Wehrlein by a full second, finishing 16th.
"In this session he's come in with a strong intent and Wehrlein is going to be the one sitting there at lunchtime thinking: 'this guy is quick'," remarked Sky F1's Paul di Resta.
The teenage Frenchman, who had previously competed in four P1 sessions for Renault, only made one error of note when he clouted a cone when returning to the pit lane.
"It was an excellent start for him," said Sky F1's Martin Brundle. "Wehrlein would have looked at [Ocon's] time and his stomach would have sunk.
"I think there's some fascinating dynamics going on, I don't think they're the best of friends those two historically, they're trying to do the same thing at the same time and one of them is going to win and one is going to lose."
Practice One timesheet
1. Nico Rosberg, Mercedes, 1:48.348
2. Lewis Hamilton, Mercedes, 1:49.078
3. Kimi Raikkonen, Ferrari, 1:49.147
4. Sergio Perez, Force India, 1:49.274
5. Sebastian Vettel, Ferrari, 1:49.768
6. Daniel Ricciardo, Red Bull, 1:49.782
7. Max Verstappen, Red Bull, 1:49.865
8. Nico Hulkenberg, Force India, 1:50.088
9. Valtteri Bottas, Williams, 1:50.394
10. Esteban Gutierrez, Haas, 1:50.583
11. Romain Grosjean, Haas, 1:50.899
12. Felipe Massa, Williams, 1:51.122
13. Marcus Ericsson, Sauber, 1:51.125
14. Carlos Sainz, Toro Rosso, 1:51.424
15. Felipe Nasr, Sauber, 1:51.768
16. Esteban Ocon, Manor, 1:51.787
17. Daniil Kvyat, Toro Rosso, 1:52.308
18. Jenson Button, McLaren, 1:52.407
19. Pascal Wehrlein, Manor, 1:52.837
20. Kevin Magnussen, Renault, 1:53.053
21. Jolyon Palmer, Renault, 1:53.089
22. Fernando Alonso, McLaren, No time