Pierre Gasly set the pace for AlphaTauri on the opening day of the final Formula One pre-season test in Bahrain.
The Frenchman vaulted to the top of the timesheets with a late-session 1:33.902-second flyer under the floodlights to end the day 0.457 seconds quicker than Ferrari's Carlos Sainz.
Charles Leclerc, quickest for most of the day with his morning best of one minute, 34.531 seconds, was relegated to third by the late improvements for Gasly and Ferrari teammate Sainz.
Most of the attention on Thursday was focused on Mercedes after their W13 challenger turned up with dramatically narrower sidepods.
But the advantage of what Formula One's motorsport managing director termed a "very extreme interpretation" of the new 2022 rules was not immediately evident with George Russell and Lewis Hamilton, who shared driving duties, ninth and 11th respectively.
Times in testing are notoriously difficult to read with teams all running their own individual programmes.
Red Bull logged the most mileage, with Mexican Sergio Perez, in the car the whole day, clocking an impressive 137 laps.
Russell and Hamilton logged a combined 121 laps for Mercedes while Alfa Romeo, who endured a difficult opening test in Barcelona last month, finally enjoyed a productive day of running with Valtteri Bottas and China's Guanyu Zhou clocking a total of 120 laps.
Ferrari, who impressed in Barcelona, continued to look strong, showing not only impressive pace but also chalking up 116 laps.
McLaren, strong in Barcelona, struggled on Thursday in Bahrain.
Lando Norris, who drove for the whole day after teammate Daniel Ricciardo felt unwell, was hampered by braking issues and managed a paltry 49 laps.
US-owned Haas were forced to sit out the morning session after their air freight, including their car, arrived late due to a technical issue with the plane carrying it.
Brazilian reserve Pietro Fittipaldi nevertheless managed 47 laps during the post-lunch session.
Thursday's running was largely incident free.
It was halted twice, first to clear up debris and again in the closing minutes when Perez beached his Red Bull in the gravel.
Teams also reported a lower degree of "porpoising", a phenomenon in which cars bounce excessively at speed, than they experienced in Barcelona, but they still have not solved the issue.
The Bahrain test concludes on Saturday.
The island kingdom will host the season-opening race on March 20.
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