Nico Rosberg lapped fastest in first practice for the Malaysian Formula One Grand Prix on Friday to maintain his 100 percent record for the season.
The Williams driver, who topped the timesheets in all three practice sessions at the season-opening race in Australia last weekend, lapped the Sepang circuit with a best time of one minute 36.260 seconds.
Japanese team mate Kazuki Nakajima was second quickest on a bright and humid morning, with both the Williams drivers popping-in their fastest laps in the closing seconds of the session.
Until then, Brawn GP's championship leader Jenson Button had looked comfortably quickest on the circuit that brought him his first podium in 2004.
Button was third fastest, in 1:36.305, with Brazilian team mate Rubens Barrichello fourth as the duo prepared for what could be their second successive one-two in Sunday's late-starting race.
The Ferraris of Brazilian Felipe Massa and 2007 world champion Kimi Raikkonen, twice a winner at Sepang including last year, followed.
Raikkonen cruised back to the pits with five minutes remaining with smoke filling the cockpit and billowing out of the rear of the Ferrari in what initial reports suggested was a problem with overheating batteries from the new KERS energy recovery system.
McLaren's world champion Lewis Hamilton was seventh quickest, a day after he was stripped of third place from Australia for "deliberately misleading" stewards at the post-race enquiry.
Team mate Heikki Kovalainen was last with just seven laps completed.
Renault's Nelson Piquet skidded into the gravel towards the end of the session while double world champion team mate Fernando Alonso, suffering an ear infection on Thursday, also ran wide.