Nico Rosberg topped the times ahead of his Mercedes team-mate and world championship leader Lewis Hamilton in Friday morning's opening free practice for this weekend's Belgian Grand Prix.
The 31-year-old German, seeking to take advantage of impending penalties that will relegate Hamilton to the back of the grid, clocked a best lap in one minute and 48.348 seconds in an uninspiring session run in hot conditions at the spectacular Spa circuit in the Ardennes.
This left him seven-tenths of a second clear of Hamilton and third-placed Kimi Raikkonen of Ferrari.
Mexican Sergio Perez, who is linked with several rival teams for 2017, was fourth for Force India, ahead of four-time champion Sebastian Vettel of Ferrari and the Red Bulls of Australian Daniel Ricciardo and Dutch teenager Max Verstappen.
The introduction of a revised power unit by Honda made an inauspicious start for McLaren with Briton Jenson Button down in 18th and team-mate Fernando Alonso unable to clock a time following a water leak.
Both Rosberg, who trails Hamilton by 19 points in the title race with nine races remaining, and Hamilton were given new Mercedes power-units for the session, a move that the team confirmed will mean Hamilton has at least a 15-place penalty on the grid.
Hamilton is expected to have another new power-unit installed for the second session on Friday and the third on Saturday morning, actions that may lead to him taking an overall record grid penalty of 75 places.
A series of engine failures early in the year meant Hamilton quickly reached the limit of new parts he is allowed for the season while Rosberg, who won the opening four races without problems, is within the limits.
Mercedes policy is aimed to be as fair as possible to both drivers and to give Hamilton sufficient new parts to last for the rest of the season with all of his penalties served at one race.
Mercedes' one-two in opening practice was their eighth of the season and 10th in 12 races.
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