Red Bull's Sebastian Vettel beat Ferrari's Fernando Alonso to set the pace
in final practice at the Belgian Grand Prix.
The world champion edged out his Spanish long-time rival by 0.105 seconds, with the second Red Bull of Mark Webber third, 0.101secs behind the Ferrari.
BBC F1 technical analyst
“Mercedes did not look happy with themselves. There were a lot of mistakes, a few lock-ups, Lewis had a spin [at Les Combes]. I think they are in a little bit of trouble”
Toro Rosso's Jean-Eric Vergne was an impressive fourth ahead of Ferrari's Felipe Massa and Jenson Button's McLaren.
Lotus's Kimi Raikkonen and Mercedes' Lewis Hamilton were only 10th and 12th.
Mercedes, who have set pole position in five of the last six races, have appeared a little out of sorts so far at Spa this weekend. Hamilton's team-mate Nico Rosberg was ninth, 0.666secs off the pace.
BBC F1 technical analyst Gary Anderson said: "Mercedes did not look happy with themselves. There were a lot of mistakes, a few lock-ups, Lewis had a spin [at the Bus Stop chicane]. I think they are in a little bit of trouble."
Vergne - annoyed that he is not in the frame for a Red Bull seat next season when his team-mate Daniel Ricciardo, who was 11th fastest is - was not the only surprise name in the top 10.
Mexican Sauber driver Esteban Gutierrez was seventh, pipping the Lotus of Romain Grosjean, who had been third fastest in the dry second session on Friday.
There were no further tyre failures following those suffered by Vettel and Alonso on Friday.
The drivers demanded an explanation, concerned because it is only three races since the multiple failures at the British Grand Prix, and Spa is an especially demanding track for tyres.
But officials believe the failures were caused by a piece of metal debris that was found at Turn 13.
It had come from Kimi Raikkonen's Lotus and its size and shape matched the damage to the punctured tyres on the Red Bull and Ferrari.
Belgian Grand Prix, day two
- Saturday 24 August : Qualifying at 12:10 BST on BBC One plus live text commentary online
Belgian Grand Prix, day three
- Sunday 25 August: Race coverage at 12:10 BST on BBC One & BBC Red Button at 13:00 BST, Radio 5 live, plus live text commentary online. F1 Forum, 15:15 Connected TV. Highlights, 19:00 BBC Three & BBC Red Button