Williams driver Valtteri Bottas set the pace from Red Bull's Daniel Ricciardo in a wet-dry final practice session at the Belgian Grand Prix.
The fastest Mercedes was championship leader Nico Rosberg in third, with Ferrari's Kimi Raikkonen splitting the German from team-mate Lewis Hamilton.
Ferrari's Fernando Alonso was sixth and McLaren's Jenson Button eighth.
"I don't think we've seen everything from Mercedes and quite a few cars," said BBC F1 analyst Allan McNish.
The session started wet after heavy overnight rain and the track was empty for more than half an hour as teams waited for it to dry.
Qualifying is expected to be wet, so teams were keen to conserve the intermediate tyre, of which they are limited to only four sets per race weekend.
"You need to save tyres and you don't want to risk burning up a set in practice. And the cars are too complex to risk a crash," said BBC F1 co-commentator David Coulthard.
The changeable conditions made it difficult to be sure that the final practice times were an accurate reflection of form, but Williams were expected to be quick going into the weekend.
Nevertheless, Coulthard said he would be "surprised if anyone other than a Mercedes was on pole".
Hamilton had a slow first sector on his fastest lap, which suggests he might have more in hand.
But Red Bull team principal Christian Horner said seeing Bottas setting the pace was "entirely predictable".
"Anyone with a Mercedes engine is going to be competitive," Horner said.
"It was a good lap at the end from Dan [Ricciardo] but it's only Saturday morning. We're just trying a few set-up things between the two cars. I'm sure they will converge for qualifying."
This weekend's race is the first after F1's four-week summer break, and the magnificent Spa track in the Ardennes mountains is a fitting stage for the latest instalment in the increasingly tense title battle between Rosberg and Hamilton.
Rosberg leads his team-mate by 11 points going into the weekend, but Hamilton appeared to have an edge on one-lap pace throughout a dry Friday, and again seemed to have a small advantage in the final session.
Bottas beat Ricciardo by 0.268secs, with Rosberg only 0.006secs further back.
After a poor season so far, Raikkonen appears back on form at a track where he has won four times and was 0.073secs quicker than Alonso, with Hamilton in between.
Only 0.428secs separated Bottas from Toro Rosso's Daniil Kvyat in seventh.