Mercedes driver Michael Schumacher set the pace in a damp second practice at the Australian Grand Prix after McLaren topped the first.
The seven-time champion, whose team are tipped to challenge in 2012, pipped fellow German Nico Hulkenberg's Force India right at the end of the session.
Earlier in the day, in a drier session, Jenson Button beat McLaren team-mate Lewis Hamilton by 0.245secs.
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BBC Radio 5 live analyst Jaime Alguersuari
“The Mercedes look impressive, but I think the Red Bulls were running a lot of fuel today. They'll show their hand tomorrow”
After a day of torrential rain and sunshine, the weekend forecast is dry.
In the afternoon session, which had only 15 minutes of dry running right at the end, Sauber's Sergio Perez was third fastest, ahead of Ferrari's Fernando Alonso, who set his time on the medium tyre, which is harder and theoretically slower than the softs used by the drivers ahead of him.
The Spaniard was also fourth in the morning session, behind Button, Hamilton and Schumacher.
World champions Red Bull had a low-key day. Mark Webber was their fastest driver in the first session in fifth, with team-mate Sebastian Vettel 11th. In the afternoon, Vettel was 10th and Webber was 11th.
But BBC 5 live analyst Jaime Alguersuari, who drove for Toro Rosso from 2009-11, said he expected Red Bull to set the pace on Saturday.
"The Mercedes looks really good," he said. "They look impressive, but I think the Red Bulls were running a lot of fuel today. They'll show their hand tomorrow."
Conditions were slower in the second session than the first, after a torrential downpour soaked the track in the break.
But both sessions were affected by the changeable weather in autumnal Melbourne - the first session also started wet but dried up quicker in the warmer early afternoon weather.
The weekend timetable is moved later in Australia than at most grands prix to ensure they are as close as possible to day time in Europe, where F1's core audience is.
Despite the tricky conditions, only two drivers spun off.
ON THE BUTTON
Button and Hamilton finished one and two in second practice for the 2011 Australian Grand Prix, this time they managed the feat in P1
Ferrari's Felipe Massa lost his car at the entry to Turn Nine in the first session after putting a rear wheel on the grass while he was braking, while Perez spun out of the final corner in the closing stages of the second session.
But several drivers had trips across the grass after missing their braking points on the slippery track.
Practice is not necessarily a true indication of competitive form, but Mercedes' performance underlined the impression that they are set for a more competitive year in 2012.
The cars were quick on both a damp track and a dry one.
Equally, Alonso's performance place suggested Ferrari might not be in as much trouble as winter testing had suggested.
Nevertheless, the car continued to look difficult to drive - and Alonso rescued a massive slide on the exit of the tricky final corner at the end of the first session, which could easily have ended with the car in the wall.
Kimi Raikkonen's running on his first day at a grand prix following a two-year spell in rallying was interrupted by a problem with his car's power steering, which lost him time in the first session.
Lowly HRT had a torrid first day of the new season with their new car.
The team had only managed 40km of running before the start of the season, and Indian Narain Karthikeyan ground to a halt out on the track after just three slow laps.
Team-mate Pedro de la Rosa managed only a single 'installation' lap as his team struggled to get his car ready.
Australian Grand Prix 2012, day two
- Saturday, 17 March: Practice 0300-0400. Qualifying 0600. Live commentary on BBC Radio 5 live. Live text and audio commentary on BBC Sport website. Extended highlights on BBC Two 1300-1415
Australian Grand Prix 2012, day three
- Sunday, 18 March: Race 0600. Live commentary on 5 live. Live text and audio commentary on BBC Sport website. Extended highlights on BBC One at 1400-1600