Max Verstappen sent out early warning signs by putting the newly-unveiled RB19 at the top of the timesheets on the first morning of F1 pre-season testing.
The reigning champion's lap time of 1:32.959 was around three tenths of a second faster than Ferrari's Carlos Sainz, who had a bizarre front wing issue in the opening hours of the day.
Both drivers registered around 70 laps of running around the Bahrain International Circuit in what turned out to be a mostly reliable first half of the day for the competitors.
Alex Albon completing a very strong opening day for the Williams team by registering the third-fastest time, and the highest lap count of the morning (74).
Alfa Romeo set the fourth-fastest time with Guanyu Zhou at the wheel, ahead of Mercedes' George Russell and F1 returnee Nico Hulkenberg, who was part of Haas' pit lane car launch at the start of the day.
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The only red flag of the day so far was caused by Felipe Drugovich, whose Aston Martin ground to a halt in the opening hour of the session with an electronics issue.
However, the Brazilian driver managed to return to the track and register 40 laps, finishing P7, and gain sharpness on the off-chance he could be called up to replace the injured Lance Stroll for the Bahrain GP.
AlphaTauri's Yuki Tsunoda finished the first half of the day in P8, ahead of Alpine's new recruit Pierre Gasly and F1 rookie Oscar Piastri, who completed 52 laps for McLaren.
With several teams swapping their drivers for the second half of test day one, take a look at who will be competing in the afternoon session here.