SPA-FRANCORCHAMPS, Belgium - Formula One leader Nico Rosberg set the fastest time in opening Belgian Grand Prix practice on Friday after renewing his duel with Mercedes team mate and title rival Lewis Hamilton.
Returning refreshed from the August break, Rosberg set a best time of one minute 51.577 seconds around the classic Ardennes circuit.
Hamilton, who is 11 points adrift after 11 of 19 races, was a mere 0.097 slower on a cloudy but bright morning at the longest track on the calendar which is renowned for fickle weather conditions.
The one-two was their fifth in succession in practice.
Ferrari's Fernando Alonso was third fastest with McLaren's Jenson Button fourth.
The Mercedes-powered teams were predicted to dominate and had seven cars in the top 10 but Ferrari's showing, with four times Spa winner Kimi Raikkonen fifth fastest despite a spin at La Source, indicated they would not have it all their own way.
Williams had a low-key start, with Finland's Valtteri Bottas 10th on the timesheets and Brazilian Felipe Massa 15th.
Red Bull's Daniel Ricciardo, winner of the previous race in Hungary, was ninth.
At the bottom end of the field, Germany's Andre Lotterer limbered up for his F1 debut with Caterham as a replacement for Japan's Kamui Kobayashi by lapping faster than Swedish rookie team mate Marcus Ericsson.
American Alexander Rossi replaced Max Chilton for the session at Marussia but his hopes of completing the weekend and racing for the first time evaporated when the Briton was reinstated following contractual wrangles.
The team gave no details but said he would be racing.