Nick Heidfeld returned to the top of a Formula One timesheet as free practice ended at the Belgian Grand Prix, taking full advantage of his position on a drying track.

The German ended the hour-long session with a lap of 1min 47.876secs as times tumbled after a slow start to get to within six tenths of Friday's pace, but completed more laps than anyone as he attempted to get his BMW Sauber fettled for the changeable conditions expected for afternoon qualifying.

The session started under a cloud, literally, and only a couple of cars - Heidfeld's and that of Sebastien Bourdais - had registered a timed lap by the time 20 minutes had elapsed. Neither could duck under 1min 59secs in that time but, as the rain abated and the Ardennes asphalt began to dry, the session gradually produced faster and faster times.

Heidfeld's countryman Timo Glock led the charge through the middle part of the session, repeatedly lowering the target time while the 'big guns' remained in the pits, but was then forced down the order as the entire field eventually poured out on track.

Points leader Lewis Hamilton then vaulted to the front, but did not last long as the benchmark continued to come down with almost every lap. Ferrari's Felipe Massa - who ran only six laps - tried and failed to dislodge the McLaren as he struggled on the damp surface, but Heidfeld, Heikki Kovalainen and Fernando Alonso all succeeded where the Brazilian failed to end the session ahead of Hamilton.

Massa's best lap, eventually eight tenths off the pace, was good enough for fifth, putting the best of the Ferraris just ahead of Toro Rosso's Sebastian Vettel, while Kimi Raikkonen, Nico Rosberg and Nelson Piquet rounded out the top ten.

Despite the tricky conditions, there were fewer incidents of note than in Friday's second practice, but one driver, Jenson Button, still failed to set a lap time as his Honda failed within yards of the Briton leaving the pit-lane, giving him no chance to improve the struggling RA108 ahead of what could be an equally challenging qualifying session.