Jenson Button's first serious drive in his new Brawn GP ended in disappointment this morning as he trailed in fifth fastest during the two free practice sessions ahead of Sunday's Australian Grand Prix.

Nico Rosberg, the Williams driver, was quickest around the Albert Park track. Lewis Hamilton, McLaren's reigning world champion, was 18th out of the 20 drivers.

Williams, Toyota and Brawn dominated the session which will infuriate their rivals because they all use the controversial rear diffusers which were deemed legal by the FIA yesterday.

Rosberg was quickest in both the opening and second practice sessions, posting a best of 1 minute, 26.053 seconds in the second round.

"It's been a positive day," Rosberg said. "We've done very well today but it's not clear how much fuel others were running.

"My hopes are higher than before that we can consistently score points. It's a nice step forward but how much, I cannot say."

Rubens Barrichello, Button's Brawn team-mate, was second fastest in the second session, ahead of Toyota's Jarno Trulli. Button said he hoped there was more to come from his car.

“I still don't think we have been able to get the best out of the car,” he said. “In a way its good that we are still not happy with the balance of the car.”

“Ferrari are competitive. Williams for sure have put in some quick lap times, and I don't know where that's come from."

Williams, who have won the drivers' championship seven times and the constructors' title nine times, have not won a race since the final event of the 2004 season at Brazil - a run of 72 races without victory. Toyota is yet to win a race since entering the sport in 2002, while Brawn took over the former Honda team after the Japanese car maker pulled out following the 2008 season.

Mark Webber steered his Red Bull to a fourth quickest time in the second session after a reliability issue in the opening stint. Button was sixth in session two, ahead of Toyota's Timo Glock, Kazuki Nakajima, the Williams driver, and Sebastien Vettel of Red Bull.

Hamilton, last year's Australian GP winner, was 16th in the first session and 18th in the second, lending credence to his pre-race predictions that his McLaren would struggle for competitiveness in Melbourne.