Jenson Button made the best possible start to his quest for a third successive Formula One victory by setting the quickest lap time in Friday's two practice sessions for the Chinese Grand Prix.
The Briton, who won the first two races of the season from pole position, again showed the pace of his Brawn GP car with a storming lap of one minute, 35.679 seconds late in the second session.
German Nico Rosberg also left it late before posting a time of 1.35.704 for Williams to edge Button's Brazilian team mate Rubens Barrichello (1.35.881) and deny Brawn GP the top two times of the day.
The Brawns were expected to dominate the weekend after their rear diffuser was declared legal on Wednesday but the Red Bulls of Mark Webber and Sebastian Vettel -- fourth and fifth quickest -- showed it was possible to go fast without one.
World champion Lewis Hamilton had put his McLaren team's troubles behind him by setting the quickest time in first practice but faded in the second session.
The 24-year-old Briton, whose title defence has been marred by struggles on the track and controversy off it, finished 13th fastest in the quicker afternoon session at the track where he claimed his last victory in the penultimate race of last season.
Ferrari's travails also continued with Brazilian Felipe Massa 12th fastest in second practice and Finn Kimi Raikkonen managing only the 14th best time for the reigning constructors' champions.
Raikkonen came off the track in both sessions, one of several drivers struggling with grip as teams weighed their tyre choices for the weekend.
A third practice session takes place before qualifying on Saturday.