Ricciardo posted his best time just over the halfway mark during the four-hour morning session, setting a 1m32.203s on hard C2 tyres in his McLaren-Mercedes MCL35B.
Alfa Romeo’s Kimi Raikkonen kicked off the first official session of F1’s 2021 season at 10:00 local time on Bahrain’s Grand Prix layout.
The dusty and windy conditions caused several drivers to lock up their wheels, with Red Bull’s Max Verstappen catching a spin as he came to grips with his revised Red Bull RB16B.
Despite the rough conditions, the morning session was relatively incident-free. Ferrari driver Charles Leclerc caused the only red flag with 10 minutes to go, parking his new Ferrari SF21 in the first sector after an apparent mechanical failure in the run up to Turn One.
Earlier, Aston Martin’s Sebastian Vettel had caused a yellow flag by grinding to a halt at the end of pitlane during the second hour.
Valtteri Bottas was on morning duty for Mercedes but hit trouble on his out-lap with the W12 with a reported gearshift issue.
The Mercedes disappeared for a lengthy spell in the garage to perform a gearbox change, but Bottas rejoined the track with 40 minutes to go the four-hour morning session.
In the opening stages, Leclerc was the first driver to post a reasonable time with a 1m37.481s, soon bested by former teammate Vettel’s 1m36.367s in the Aston Martin AMR21 and then Raikkonen’s 1m34.686s in the Alfa Romeo.
Alpine’s Esteban Ocon and Verstappen further lowered the benchmark until Ricciardo grabbed the lead in the second hour with a 1m32.537s on medium C3 tyres.
At the halfway point, AlphaTauri driver Pierre Gasly briefly took the lead with a 1m32.525 on C2 tyres, but Ricciardo immediately reclaimed the top spot on the timesheets with a 1m32.203 jut 0.028s ahead of Gasly and 0.042 clear of Verstappen.
Ocon posted the fourth-fastest time for the rebranded Alpine team some seven tenths down on Ricciardo.
Leclerc claimed fifth for Ferrari, one second behind Ricciardo, closely followed by Raikkonen in sixth and Vettel in seventh.
Williams test driver Roy Nissany finished in eighth place in the Williams FW43B, while Mick Schumacher ended up in ninth after a late timed lap in the Haas after a disrupted morning.
Haas had to park Schumacher to replace the Ferrari gearbox in the back of the VF21, limiting the German rookie to just 15 laps.
Bottas and Mercedes finished tenth and last, 4.647s behind Ricciardo, being restricted to just six laps after their early gearbox issue.
Gasly was the most productive driver of the bunch, leading the lap chart with 74 tours.
Testing continues at 15:00 local time for another four-hour session, although predicted sandstorms are expected to worsen conditions in the afternoon.
F1 teams get two more days of testing on Saturday and Sunday ahead of March 28’s season-opener at the same venue.