Red Bull's Mark Webber set the fastest time in final practice for the Belgian Grand Prix.
The championship leader lapped in one minute, 46.106 seconds shortly before heavy rain forced the drivers to switch back to wet tyres.
McLaren's Lewis Hamilton was second fastest, with Webber's Red Bull team-mate Sebastian Vettel third.
Vettel was followed by McLaren's Jenson Button, Renault's Robert Kubica and Ferrari's Fernando Alonso.
Only half a second covered the first six drivers, with Alonso's team-mate Felipe Massa in seventh and Force India's Adrian Sutil again showing encouraging form to be eighth fastest.
After the wet weather of Friday, the track had dried sufficiently to give all the teams valuable running time on slick tyres from the start. With rain forecast, all took advantage, with Hamilton setting a succession of fastest laps on his prime rubber.
It was again notable that the Red Bulls, though faster in the twistier middle sector of the track, had less of an advantage in the flowing first and third sectors.
With 20 minutes remaining most the drivers switched to their softer option slick tyres, but the rain that had been threatening began to fall.
As it did so, Vettel stopped in the pit-lane and had to be pushed back to the garage, but the airbox problem that was diagnosed was almost immediately fixed.
The German is the only one of the leading contenders to be using an old - previously used - engine at a track where power is vital.
On Friday, meanwhile, it was confirmed that Red Bull had passed new, tougher front wing flexibility tests introduced in an effort to allay concerns that their pace advantage may be at least partly down to illegally flexing bodywork, particularly the front wing and forward part of the under-floor.
Mark Webber's RB6, and Lewis Hamilton's McLaren MP4-25, were subject to the new tests during the standard inspection procedure that followed Friday's practice sessions.
Previously, the endplates on the front wing were allowed to flex by a maximum of 10mm when a load of 50 kilogrammes was placed upon them. The new test saw that weight doubled to 100 kgs, with deflection allowed of up to 20mm.
According to FIA technical delegate Jo Bauer, both cars - which were also subject to an additional front floor deflection test - were found to be in conformity with 2010 technical regulations.
Additional new floor tests and regulations will be introduced at the Italian GP in two weeks time.
Red Bull have always been adamant their cars comply with the regulations. Neither Ferrari was chosen for testing on Friday, but Scuderia technical director Aldo Costa said he hoped the FIA's stronger stance would end the controversy.
