Heidfeld not judging BMW form yet

"I still find it very difficult to make a judgement on everybody's performance," said Heidfeld, who finished down in 12th position.

"I'm afraid we have to be patient until tomorrow, at least until the second part of qualifying.

"I assume some of the cars which are higher up on the result sheet were lighter than us. I had no technical problems in the second session, and the failure in the morning just cost one run which isn't a drama."

The German squad, aiming to take their first win this season after being the third strongest outfit last year, failed to impress in today's sessions, with Heidfeld's teammate Robert Kubica well down the order in 15th.

The Pole said he was suffering from tyre degradation, but had a trouble-free day.

"As well as working on the set-up of the car, we concentrated mainly on comparing the two tyre compounds," he said.

"Degradation is quite a big issue in these hot conditions. We have to analyse the data and find out how best to exploit the full potential of the tyres."