Red Bull held their edge over McLaren as Mark Webber led Jenson Button and Lewis Hamilton in practice for the Malaysian Grand Prix.
Webber, who took pole at Sepang's high-speed track in 2010, eclipsed Button by 0.005 seconds, with Hamilton's McLaren 0.134secs adrift.
World champion Sebastian Vettel was fourth ahead of Mercedes' Michael Schumacher and Ferrari's Felipe Massa.
The teams will fine-tune their form in Saturday's final practice at 0700 BST.
"It's good to see Red Bull aren't just walking away with it," commented BBC Radio 5 live analyst Anthony Davidson.
"They still have the advantage at this stage, and you can be sure that they are not running around on a light fuel load.
"I think Vettel has a lot more up his sleeve for qualifying, and that's when we'll see the real result."
Webber put the disappointment of finishing fifth at the opening Australian Grand Prix behind him by topping both practice sessions in Malaysia.
The Australian took pole at Sepang's fast-and-flowing circuit last season, but conceded victory to his team-mate Vettel after he made a mistake at the start.
Webber comfortably outpaced Hamilton's McLaren in the morning session by 1.6secs, but Button closed the gap considerably in the afternoon.
McLaren were optimistic about their chances of challenging Red Bull at Sepang, and the team were again the champions' closest rivals.
Both Hamilton and Button complained about a lack of balance and grip in the morning, but the team set about making improvements during the break.
Vettel did not show his hand in practice, and the championship leader was content to run a low-key session on the track where he recorded his first victory of 2010.
"I think Red Bull are always blowing smoke up other people," said Hamilton. "I'm sure they have got half a second at least to pull out tomorrow.
"Red Bull are using Kers now this weekend, so that cancels the advantage that we may have had. The track conditions are massively different here [to Melbourne] - we have 50 degree track temperatures so the tyres aren't lasting as long."
Button added: "We set the car up reasonably similar to Melbourne and it took the car in one direction. But we've turned the car upside down and it's performing much better now on the short runs and the long run at the end.
"Rear grip is very difficult around here. When the soft tyres drop off they fall off a cliff, so it's very tricky."
Ferrari also chose not to focus on performance during first practice, and instead ran through the aerodynamics of the car on a track where efficient downforce is critical to overall performance.
But after saying they were to spend the second session in a more conventional fashion chasing race set-up, they admitted on the Twitter social networking site that they were "far away from the leaders", describing it as "a difficult Friday".
Massa finished the day sixth, 1.213secs off Webber's pace, with Alonso back in ninth, 1.707secs adrift of the top time.
Alonso said: "We struggled a little bit with the balance of the car.
"Obviously we know that we need to keep working tonight, to keep changing what we can improve in the car, and be more competitive tomorrow."
"I think the race weekend is very interesting, what we have in front of us.
"A very interesting qualifying then a difficult race for everybody here, with pit stops, with tyre degradation that seems to be higher here than Melbourne, with the weather conditions we can face on Sunday.
"This is only the start of the season: we are not quick enough to fight for the win and pole position, but that does not mean that we have to throw in the towel."
The demanding Sepang track and the gruelling temperatures took their toll on the Formula 1 field on Friday.
Renault were aiming to build on Vitaly Petrov's third place in Melbourne this weekend, with team boss Eric Boullier saying the team could go even faster.
But the team's preparations were hit when both Nick Heidfeld and Petrov both exited first practice in spectacular fashion.
Heidfeld had to nurse his Renault back to the pits after the brakes jammed on his front-right wheel, and things got worse when Petrov ploughed his car into the gravel.
The team spent their lunchtime investigating potential problems with the upright, which holds the brake and wheel in place, and Renault did well to get both cars out in the afternoon.
Team principal Eric Boullier said: "It took a long time to investigate but we had a couple of upright failures. They were the same uprights [as used in Australia] but a different material. It looks like a heat treatment and material issue. We had a back-up plan."
Schumacher's fifth place, together with seventh for Nico Rosberg, has buoyed Mercedes after their disappointing weekend at the Australian GP.
Team principal Ross Brawn said: "The car isn't where we want it to be yet, but it's a lot better than it was in Australia.
"We have a little while to go before we trouble Red Bull, but running the car as consistently as we did today means we can understand it a bit better."
Pastor Maldonado held the fastest time fleetingly in the afternoon when he was the first man to run on the faster soft tyres, but he ruined his day as he put two wheels on the grass and spun 360 degrees before crunching his Williams into the tyre wall.
The team did a solid job to get the Venezuelan rookie back out on track.
After failing to qualify for the Australian Grand Prix under the re-introduced 107% rule, Hispania had also been confident of a more positive weekend in Malaysia.
But the Spanish team's hopes literally went up in smoke in first practice as Narain Karthikeyan had only just nosed out of the garage to set out on his third lap when he sent thick plumes into the air.
There was more smoke from Karthikeyan's Hispania in the afternoon session, while team-mate Vitantonio Liuzzi was visibly manhandling his car before he ended up on the grass to complete another frustrating day for the team.
However, there was a chink of light in that Karthikeyan did manage to sneak inside 107% of Webber's fastest time.
The Indian said on Twitter: "Feels good to get some running. Lot of work to be done, but feel positive for qualifying! Glad to be within the 107%."