Formula One leader Jenson Button of Brawn GP was fastest in the first practice session for the Spanish Grand Prix.
The British driver set a fastest lap time of 1 minute, 21.799 seconds this morning at the Circuit de Catalunya.
Toyota's Jarno Trulli was second with a best lap of 1:22.154, while BMW Sauber turned in much-improved performances as Robert Kubica and Nick Heidfeld came next.
Kazuki Nakajima of Williams rounded out the top five.
Button had earlier told reporters that despite three wins in four starts, it was still far too early to talk of the title and he would not be taking a strategic approach to Sunday's race.
"It's so important to be reliable, but ... you've got to be very aggressive at this point in the year to get as many points as you can. You can't settle for a second or third. We don't have enough of a lead yet," he said.
"It might all go wrong, but I think you need to be aggressive because if you potter around and pick up the points, you haven't got a chance when it gets to the end of the year."
The Briton, who has a 12-point cushion over Brazilian team mate Rubens Barrichello, is revelling in his new status as championship frontrunner after being widely written off at the end of last year when Honda announced they were pulling out.
Brawn, with a Mercedes engine, have turned Formula One upside down since they emerged from the embers of the Japanese team.
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The 29-year-old Button won the last race in Bahrain from fourth place on the starting grid after a fighting first lap in which he was passed by McLaren's world champion Lewis Hamilton and then retook the position.
Before that, he won in Australia and Malaysia, where he earned only half points after the race was stopped by heavy rain, and was third in China behind the two Red Bulls.
"You have to be fighting for a win at every race you go to," he said. "If I didn't in Bahrain, we probably would have finished second or third and missed valuable points," he said.
"You have one reliability issue and you've lost 10 points, then I've got a two point lead and that's it. It can turn around so quickly, we've seen it many times."
Button said the first race in Europe this season was an important one for all the teams, with the sport's misfiring big guns all bringing major new aerodynamic upgrades to the Circuit de Catalunya.
So too do Brawn, however.
"We've got a new package here, and we won't be getting a new package immediately after, so this package has to give us a good lump in lap time," said the Briton, whose team were very quick in pre-season testing in Barcelona.
"We have to hope that ours is enough to put us in front of the Red Bulls by a small margin, because I'm sure they're going to be improving all the time.
"This is a big aero circuit, and aero-wise we are good, but we're not as good as people think we are," he added.