Williams have proved the surprise of the first official Formula One practice session of 2009, dominating with a late run ahead of Sunday's Australian Grand Prix.
Nico Rosberg took his Williams around Albert Park in one minute 26.687 seconds - four hundredths of a second quicker than his teammate Kazuki Nakajima.
Ferrari and the new Brawn GP team had held the lead throughout most of the session before being toppled by the Williams cars.
Ferrari's former world champion Kimi Raikkonen set a quick lap early but found himself relegated to third, followed by the first Brawn GP driven by Rubens Barrichello.
Jenson Button was sixth quickest in the other Brawn while the d efending champion Lewis Hamilton struggled and was 2.3 seconds back
His teammate Heikki Kovalainen, was closer to the pace though running fifth, less than a second from the lead.