After a four-week hiatus, F1 action returned at Spa-Francorchamps on Friday, with Mercedes once again dominating proceedings.
Championship leader Rosberg, who holds an 11-point advantage over team-mate Lewis Hamilton, showed good early pace and clocked the fastest lap of one minute 51.577 seconds.
Hamilton was second, 0.097secs slower than the German, while Ferrari's Fernando Alonso was 0.228secs off Rosberg's pace in third.
Alonso's team-mate Kimi Raikkonen, who has won four times round this circuit, was fifth-fastest, with McLaren's Jenson Button sandwiched between the two Ferrari men in fourth.
Defending champion Sebastian Vettel of Red Bull was 11th fastest, 1.792secs behind Rosberg.
Vettel's team-mate Daniel Ricciardo - the only non-Mercedes driver to have won a race this season - once again went quicker than the German, but could only record the ninth-fastest time, behind the two Force Indias of Sergio Perez and Nico Hulkenberg, as well as Kevin Magnussen of McLaren.
There was a twist in the story at the Marussia garage, after it was revealed that Max Chilton will race on Sunday, despite reserve driver Alexander Rossi taking the car in practice.
On Thursday, it was announced that Rossi would take Chilton's place as the team looked to sort out "contractual issues" with the Briton. But as Rossi drove to a 20th-placed finish on Friday, it was revealed there had been a U-turn and that Chilton will be the man to race come Sunday.