Lewis Hamilton headed Nico Rosberg to a Mercedes one-two in second practice at the season-opening Australian GP.
Their performance underlined their status as favourites for this year's championship as Ferrari's Fernando Alonso took third.
Despite Red Bull's troubled pre-season programme, Sebastian Vettel was fourth from McLaren's Jenson Button.
Vettel's team-mate Daniel Ricciardo was sixth from Ferrari's Kimi Raikkonen and Williams' Valtteri Bottas.
Raikkonen, whose partnership with Alonso is one of the most eagerly anticipated aspects of the new season, was 0.7 seconds slower than the Spaniard.
Ferrari team principal Stefano Domenicali said Raikkonen had had a problem with his car, without defining what it was.
Hamilton was 0.157secs clear of Rosberg despite missing the whole first session when the car stopped with an oil sensor calibration problem on his first serious lap.
"I went out a lot later than when the others did their option [tyre] running so the track would be faster," said Hamilton. "It didn't feel like a particularly good lap. I still have a lot to improve on, a lot to find.
"P2 was good but it doesn't make any difference whether you're at the top of the time sheets in practice. The big thing was to just feel the car. Missing P1 was a big backwards step but I felt we recovered most of it."
Rosberg was in turn 0.35secs quicker than Alonso, who was fastest in the first practice session.
That is a smaller margin than was expected after a pre-season in which Mercedes and their customer team Williams were believed by their rivals to have a significant advantage over the rest of the field.
Red Bull's performance proved they have made significant progress since the final pre-season test two weeks ago.
The car ran mostly reliably, although Vettel did have to stop with what appeared to be an overheating problem as he returned to the pits towards the end of the first session.
"It's been a very strong first day for us," said Red Bull team principal Christian Horner. "We've had a very difficult pre-season with numerous issues so it was refreshing to get some laps in and get a first proper feel for the car.
"We can see there is quite a gap to the Mercedes and the Ferrari, but we are starting to feel what that gap is and by the end of the weekend we will have a clearer idea."
The Red Bulls appeared also to be relatively competitive in race trim - but Mercedes look to be in a league of their own both on single-lap pace and in race trim.
Rosberg and Hamilton were at least half a second a lap quicker than their rivals in the race-simulation runs in the second part of the session.
BBC F1 analyst Allan McNish said: "The Mercedes is very quick, but the battle for second is close between McLaren, Williams, Red Bull and Ferrari.
"I think Ferrari will be there or thereabouts, too, but I am a little bit confused as to why Alonso appears to be able to get more out of the car than Kimi Raikkonen.
"The stand-out story, though, seems to be that Lotus are struggling. They over-performed last year but have not done much running and when the car is out [on track] it looks unstable."
Lotus' Romain Grosjean was only 18th, four seconds off the pace. Team-mate Pastor Maldonado did not even set a time.