World championship leader Nico Rosberg topped the times just ahead of his Mercedes team-mate and nearest rival Lewis Hamilton in Friday's opening free practice for Sunday's Belgian Grand Prix.
Rosberg, who leads Hamilton by 11 points in the title race, clocked a best lap of one minute and 51.577 seconds around the spectacular and sprawling 7.004-km circuit set in the Belgian Ardennes.
This gave him an advantage of just 0.097 seconds and confirmed the ferocity of the close contest between the two as they rejoin battle a month after their controversial disagreement at the Hungarian Grand Prix where Hamilton ignored team instructions to allow Rosberg to pass him.
This was the pair's fifth consecutive one-two domination of opening practice.
Two time former champion Fernando Alonso of Ferrari was third fastest ahead of Jenson Button of McLaren and Kimi Raikkonen, who has won four times in Belgium, in the second Ferrari.
Sergio Perez was sixth for Force India ahead of Danish rookie Jan Magnussen in the second McLaren, Nico Hulkenberg in the second Force India and Australian Daniel Ricciardo, winner in Hungary, in the leading Red Bull.
Finn Valtteri Bottas was 10th for Williams with four time world champion Sebastian Vettel 11th in the second Red Bull.
The morning's practice took place in dry if cold conditions at the majestic old circuit which was first used for racing in 1922, when much of it consisted of public roads.
It remains one of the fastest circuits in use on the F1 calendar and still includes three of the most challenging corners - in Eau Rouge, Pouhon and Blanchimont.