Charles Leclerc topped the first session of the Bahrain pre-season test as teams continued to rack up mileage ahead of the new campaign.
The Ferrari driver set a lap time of one minute, 34.531secs around halfway through the morning's running on his way to completing 64 laps.
The main focus of the first four hours has been the radical upgrade to Mercedes' sidepods, with the W13 featuring what Ross Brawn has described as "disappearing sidepods".
After the first test in Barcelona, which was largely used as an extended shakedown for teams, upgrades to cars were added in preparation for next weekend's first race of the year at the same venue.
But the field will have to wait a little longer for high-performance running and with it, the first hints of a pecking order.
Alex Albon finished the morning second fastest for Williams, half-a-second slower than Leclerc.
Lewis Hamilton added 61 laps to his name in the eye-catching Mercedes, almost two seconds down on the pacesetter.
Red Bull enjoyed a healthy morning as Sergio Perez completed 69 laps, with all of the team's drama coming off the circuit as team principal Christian Horner sparked debate in the paddock with an 'unofficial' comment on the legality of Mercedes' design.
Sebastian Vettel finished third and completed 38 laps for Aston Martin, despite a number of curious escapades as a result of brake lock-ups.
In fact, the only major issue experienced all morning was by McLaren, with work taking place around the brakes in the last hour of the session. Lando Norris completed just 21 laps having taken duties over from the unwell Daniel Ricciardo.
Esteban Ocon got Alpine back on track after the team's fiery end to the Barcelona test with 42 laps, whilst Guanyu Zhou continued his preparations for his rookie season with 53 laps under his belt for Alfa Romeo.
Pierre Gasly was the slowest of the morning's runners, completing 44 laps in the AlphaTauri AT03.
Haas test driver Pietro Fittipaldi will emerge in the afternoon with the team missing the morning due to freight delays earlier in the week.
Bahrain pre-season test - Day one morning results
1. Charles Leclerc [Ferrari] - 1:34.531 [64 laps]
2. Alex Albon [Williams] - +0.539secs 
3. Sebastian Vettel [Aston Martin] - +1.175s 
4. Sergio Perez [Red Bull] - +1.446s 
5. Lewis Hamilton [Mercedes] - +1.834s 
6. Esteban Ocon [Alpine] - +2.237s 
7. Guanyu Zhou [Alfa Romeo] - +2.633s 
8. Lando Norris [McLaren] - +3.049s 
9. Pierre Gasly [AlphaTauri] - +3.357s 
10. Pietro Fittipaldi [Haas] - No time