Belgian Grand Prix: Nico Rosberg fastest after first practice at Spa
Nico Rosberg was comfortably fastest from Mercedes team-mate Lewis Hamilton in a low-key first practice session at the Belgian Grand Prix.
The German was 0.73 seconds quicker than Hamilton, who was just 0.069secs ahead of Ferrari's Kimi Raikkonen.
Mercedes were the only team to run the fastest tyre, the super-soft, as the team plan a weekend on which they know Hamilton will start from the back.
The world champion has a grid penalty for using too many engine parts.
So far, a 15-place penalty has been confirmed after Hamilton took a sixth turbo and MGU-H, the motor-generator unit that recovers energy from the turbo - one more than the number of parts each driver is allowed to use in a season.
However, it is likely that as the weekend develops Mercedes will give him a seventh of each part, adding a second 15-place penalty to put him right at the back, and possibly further new parts too, as they seek to take in one hit all the new parts Hamilton will need for the remaining nine races of the season.
Hamilton starts the weekend - the first after F1's summer break - with a 19-point advantage over Rosberg and say he is planning a race of "damage limitation" to his title hopes.
Problems for Alonso
There were also engine-related issues for McLaren, with Fernando Alonso not able to set a time because of a problem with the upgraded specification of engine Honda has introduced for this weekend.
The engine is being changed for Friday afternoon's second session and Honda have not yet revealed whether they will fit the car with a previously used engine, which would not incur a penalty, or a second version of the new one, which would.
The upgrades are in the internal combustion engine, turbo and compressor, and are said to be worth 0.1-0.2secs a lap.
Alonso's team-mate Jenson Button was only 17th fastest, 4.059secs off the pace.
At the front, Raikkonen's pace was encouraging for Ferrari, as he ran only the soft tyre, which is about the same margin slower than the super-soft as he was behind Rosberg.
Behind the Mercedes drivers and Raikkonen, Force India's Sergio Perez was fourth, ahead of the second Ferrari of Sebastian Vettel.
Red Bull's Daniel Ricciardo was sixth, his team-mate Max Verstappen vaulting up behind him ahead of Williams' Valtteri Bottas with a lap right at the end of the session.
Verstappen had an eventful session, with one huge oversteer moment at the Fagnes chicane, which he managed to catch, and a number of other twitches.
Haas driver Romain Grosjean had an off into the gravel at the Stavelot right-hander at the end of the middle sector of the lap in the closing minutes of the session.
Halo on trial
There were precious few other incidents in the session as the second half of the season got off to a slow start on a gloriously hot and sunny day at a track more famous for its capricious weather.
Rosberg was one of several drivers to trial the 'halo' head protection system at the start of the session - and actually set his fastest time with the device fitted to the car.
Others to run it were Red Bull's Daniel Ricciardo, Toro Rosso's Carlos Sainz and Force India's Nico Hulkenberg as F1 bosses assess its impact on visibility before the introduction of head protection in the 2018 season.
Strong debut from Ocon
Down the field, there was a highly impressive debut performance for Esteban Ocon, the Frenchman who has replaced Rio Haryanto at Manor after the Indonesian pay-driver's funding ran dry.
Ocon, 19, was 15th fastest, five places and the best part of a second faster than team-mate Pascal Wehrlein, who has been driving the car all season. Ocon ran the time on the 'soft' compound Pirelli test tyre, while Wehrlein's was on the medium.
Ocon is Renault's reserve driver, but is under contract to Mercedes, who are effectively staging a shootout between their two proteges to see who is the best prospect for the future.
Ocon's only error in an accomplished performance was hitting a cone in the pits as he practised a stop.
Englishman Jolyon Palmer was 21st, and slowest of the drivers who set a time, after his session came to an early end with 20 minutes to go when he lost drive. But he still ended up just 0.036secs behind team-mate Kevin Magnussen even though the Dane set his fastest time on the soft tyre and Palmer on the medium.