Sergio Perez topped the times for Red Bull ahead of Fernando Alonso, debuting for Aston Martin, as the defending champion team set the pace in Friday's opening practice session at the season-opening Bahrain Grand Prix.
The 33-year-old Mexican, who was fastest on the final day of pre-season testing at the same venue, clocked a best lap in one minute and 32.758 seconds in very warm conditions.
Alonso confirmed his promising pace from testing with a lap in 1:33.196 to split the two Red Bulls and push two-time defending world champion Max Verstappen down to third, six-tenths of a second off his teammate's time.
Lando Norris was fourth for McLaren, a further eight-tenths adrift, ahead of Charles Leclerc of Ferrari and Lance Stroll, back after recovering from a double wrist injury, in the second Aston Martin.
Kevin Magnussen was seventh for Haas ahead of Zhou Guanyu and his Alfa Romeo teammate Valtteri Bottas with seven-time champion Lewis Hamilton finishing 10th for Mercedes.
The new season began in earnest at the Bahrain International Circuit at Sakhir, a former camel farm in the desert.
Two-time champion Alonso, recruited from Alpine, to replace retired four-time champion Sebastian Vettel, set an early marker in 1:35.048.
That lap confirmed the level of high confidence in the Silverstone-based team this year, following a strong showing in last week's three-day test.
He was soon overhauled, after 14 minutes of the opening hour's free practice, by Perez clocking 1:34.343 on medium tyres, lifting him half a second clear at the top.
'DISFIGUREMENT'
Hamilton, reprising his use of a rainbow-coloured helmet in recognition of the Progress Pride flag to support diversity and the LGBTQ+ community, slotted in third behind Alonso.
Hamilton on Thursday told reporters he would continue to speak out on all issues despite a revision of the ruling body's sporting code requiring written permission to comment on political, religious and personal interests.
"I have always felt a responsibility and if the sport is going to these countries, we are duty bound to raise awareness and try to leave a positive impact," he said.
The Mercedes driver was granted an exemption before practice from the FIA's 'bling ban' after doctors' examinations supported a fear of 'disfigurement' if he was forced to remove his nose stud frequently.
Stroll's progress -– he hoped to make up for lost time when missing the testing session –- was restricted by an ignition problem to only two laps, but swift repairs returned him to the track.
After half an hour, he was back in the groove as most teams switched to soft tyres, Perez trimming his time to 1:32.758 ahead of Verstappen in 1:33.375.
Behind the two Red Bulls, Bottas was third for Alfa Romeo, but adrift by 1.3 seconds -– an ominous gap even so early in the new campaign, though Ferrari and Mercedes were still on mediums.
In pursuit, Ferrari's Carlos Sainz promptly spun on a bump at Turn Nine at high speed, flat-spotting his medium tyres before returning to the pits.
More evidence of Alonso's pace came with nine minutes remaining when he split the two Red Bulls to take second in 1:33.196 on softs, drawing an appreciative nod from Martin Whitmarsh, the former McLaren boss who is now Aston Martin Group chief executive.
Stroll then went sixth, a second away from Alonso, but confirmation of his fitness and the team's promising early pace.