Max Verstappen topped the times and then tried to play down his early advantage over Lewis Hamilton after Friday's practice for this weekend's season-opening Bahrain Grand Prix.
The Dutchman, who was fastest in pre-season testing and quickest again in both sessions, avoided showing any over-confidence after a closely-fought session that saw him finish narrowly ahead of McLaren's Lando Norris and seven-time champion Hamilton in the Mercedes.
"It was alright," he said. "Of course, the conditions are different compared to testing,but it's alright. I have a good feeling with the car and we still have things to look at tyre-wise and balance, but overall I'm happy.
"Tomorrow (for qualifying) is a different day – and it will be even windier, which with these cars makes it quite tricky."
The Red Bull driver outpaced nearest rival Norris, in his Mercedes-powered McLaren, by 0.095 seconds and Hamilton by slightly more than two-thirds.
Ferrari new boy Carlos Sainz, in his first day's work since joining from McLaren, was fourth ahead of Valtteri Bottas in the second Mercedes with Daniel Ricciardo, on his first outing for McLaren after leaving Renault winding up sixth, less than four-tenths off the pace.
"We thought that Red Bull would be as fast as they are, if not faster," said Hamilton. "So, we know they're leading at the moment.
"McLaren are looking great and it's great to see them taking the step. It will be interesting to see how their long runs are."
For Hamilton, the prosect of stronger changeable winds was not welcome.
"It's not great for us," he added. "The rear is unstable as it is, but everyone, I'm sure, will be in the same boat -- some more, some less."
"For people watching and trying to understand it's when you approach a corner and the rear moves."
- Seventh for Tsunoda -
Mercedes' lack of rear end stability followed aerodynamic regulations changes – which involved cutting away a segment of the floor close to the rear wheels -- that hit their low-rake design more than rival teams.
Aston Martin, so impressive last season when called Racing Point, were also hit by this issue and have been working feverishly to improve their floor and diffuser performance.
Team boss Toto Wolff said Mercedes had reduced the problem since the pre-season test, but Bottas was still struggling so much he described his car as "undriveable at times".
On a hot day at the Bahrain International Circuit, Japanese rookie Yuki Tsunoda was an impressive seventh for AlphaTauri ahead of Lance Stroll, of Aston Martin, Pierre Gasly in the second AlphaTauri and new recruit Sergio Perez in the second Red Bull.
But the one-hour session, reduced from 90 minutes to cut costs, provided few clear signals for the season ahead other than confirmation that Red Bull are narrowly fastest ahead of a tight pack.
In the second session, run in cooler conditions under lights, Hamilton was soon on top ahead of Bottas before Kimi Raikkonen, at 41 the oldest man in the field, spun his Alfa Romeo at Turn Three, bouncing through the gravel and hitting the wall.
The Finn's accident ripped off the nose and smacked his right rear wheel into the barrier.
A strong swirling wind, which had changed direction in the cooler temperatures, may have helped cause his mishap.
Hamilton then switched to softs with a group of others and continued to lead the way after overhauling Sainz, who had gone top, before Norris took control only to be beaten by Verstappen in 1:30.847.
With 15 minutes remaining, there was less than a second separating the top 15 cars – a sure sign that the teams are tantalisingly close in basic performance, including two-time champion Fernando Alonso, in Renault's re-branded Alpine, down in 15th, in the final season under the current regulations.
Mick Schumacher, the son of seven-time world champion Michael, clocked 1:33.297 in his Haas to be in 18th place in the 20-man field.