Charles Leclerc set the pace on his final F1 pre-season outing for Ferrari during a session in which there were three red flags.
Question marks have been raised over the three days of testing at the Bahrain International Circuit as to the competitiveness of the SF-23, particularly given the imposing form of Max Verstappen and Red Bull.
Leclerc, last season's distant runner-up to Verstappen in the drivers' standings, at least responded during the morning session at the Sakhir venue ahead of handing the car over to team-mate Carlos Sainz.
The Monégasque driver posted the quickest lap of testing to date with a time of 1:31.024s on the C4 tyre, one of the two soft-tyre compounds available to the 10 teams from supplier Pirelli.
That was 0.418s quicker than Mercedes' George Russell on the C5 – the softest rubber – as the team recovered from its issues on Friday when the Briton stopped on track during the afternoon-evening running with a hydraulics failure.
As for the trio of stoppages in Saturday's initial session, the first of those was for Sergio Perez shortly after the green light was shown at pit exit.
Just seven minutes of the day's running had elapsed when a sensor on the Red Bull's rear wing worked loose and fell onto the track.
Though off the racing line, the session was interrupted to retrieve the debris while Perez returned to the garage without further issue.
Perez went on to finish fourth quickest behind Leclerc, Russell, and the Aston Martin of Felipe Drugovich, with the Mexican driver 1.435s off the pace but due to run again in the second session.
Valtteri Bottas caused the second red flag with an hour and 40 minutes remaining as his Alfa Romeo ground to a halt on track, requiring it to be retrieved.
Bottas had completed 72 laps by that stage but found himself confined to the garage for the remainder of the session, leaving the Finn down in ninth of the 10 drivers on the timesheet, 5.830s behind Leclerc.
AlphaTauri's Nyck de Vries was over seven seconds back albeit clearly focusing on long-run pace, clocking up the morning high of 87 laps.
Behind Perez was Alpine's Pierre Gasly, the Williams of Alex Albon and Nico Hülkenberg in his Haas.
As for McLaren, it was another worrying session for the team, with Oscar Piastri on the sidelines for a short period as the team was forced to conduct strengthening work on the front winglets to ensure he could complete the programme.
The Melburnian finished eighth quickest, with the best of his session-low 45 laps a 1:33.655s.
There was a final red flag with 10 minutes remaining but that was a planned stoppage, following which Piastri and Russell conducted practice starts.
Results: F1 pre-season testing (Saturday morning)
|Alfa Romeo Sauber
|Nyck de Vries