Jenson Button and Rubens Barrichello have admitted that Brawn GP has work to do in Saturday practice after ending the first day of running for the Belgian Grand Prix at Spa down towards the bottom of the times.

Points leader Button, heading to Belgium after a second successive seventh place finish last weekend in Valencia, posted the second quickest time during the morning session - which started dry before rain hit the Ardennes and rendered the times through the remainder of the session meaningless.

Having completed wet weather testing during the morning, Button was able to then make the most of dry conditions in the afternoon to work through a programme of aerodynamic and downforce work, although the Briton conceded that there were areas for improvement after managing only the 17th best time.

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"It was a tough session this morning as we weren't able to get much done apart from some wet running at the end," he said. "However that experience is good here just in case it rains over the weekend, which is always possible at Spa. The second session this afternoon was much more useful. We worked through a few aero and downforce comparisons as well as looking at the tyres.

"We collected a lot of information but obviously at the moment we're not as quick as we would like to be. There are a few areas that we do need to improve on with the balance but the potential is there to do that ahead of qualifying."

Team-mate Barrichello ended up just behind Button on the timesheets, and while the Brazilian admitted that the car didn't feel as strong as in Valencia, he was keen to play down the importance of Friday lap times.

"We got through a lot of work today which sets the weekend up nicely," the European GP victor said. "There is still some way to go to realise the full potential of the car but as always on a Friday, the lap times don't tell the whole story.

"It was pretty useful to get some wet running in this morning just in case the weather plays a part in qualifying or the race. The car doesn't feel as competitive as it did in Valencia last weekend but we're going to be in a good position to fight hard in qualifying tomorrow."