Pierre Gasly set the pace on the opening day of official F1 pre-season testing in Bahrain, with the AlphaTauri driver putting in a 1:33.902 during a flurry of fast laps in the final hour of track time, as temperatures cooled at the Sakhir circuit.
With the afternoon session running into dusk, Gasly made the most of the cooler temperatures to fit a set of Soft C5 tyres. Initially putting in a 1m34.0s lap, this was already enough to take over at the front, but the Frenchman regrouped and went quicker again as the final half hour began.
However, while the AT03 had the ultimate pace on the day, the car clearly struggled with 'porpoising' as Gasly bounced his way down the straights.
Carlos Sainz, taking over the Ferrari F1-75 for the afternoon after Charles Leclerc went fastest in the morning session, rose to finish second as he also put in his best time late in the evening. His effort was almost half a second slower than what Gasly managed.
Leclerc, who sat firmly at the top of the times for most of the day and fastest in the morning session, finished in third place, a further tenth and a half back from Sainz.
Meanwhile, it was a quiet day for Red Bull, as Sergio Perez placed 10th, with his best time being two seconds away from the front. The Mexican driver also brought out the red flag with seven minutes remaining in the day, having spun off at Turn 8 after losing the rear of the RB18.
Mercedes stay under the radar
Mercedes, which showed up with a radically different W13 that boasted significantly streamlined sidepods and caught the attention of many in the paddock, had an otherwise low-key day.
George Russell, who drove for the afternoon, finished ninth and appeared to struggle for ultimate braking power as he ran wide on several occasions. Lewis Hamilton, driving in the morning, could only manage 11th, a further four-tenths slower than what Russell managed.
Meanwhile, at Haas, the American-led team managed to get their VF-22 on track after a logistics issue prevented them from arriving in Bahrain in time. After the lunch break, Pietro Fittipaldi took to the track and put in the 15th, and slowest, time of the day.
The day's other red flag was caused by Aston Martin, after Lance Stroll scattered the remains of a loose aerodynamic rake across the track early in the afternoon session.