Pierre Gasly topped the first day of F1 pre-season testing in Bahrain but it was Mercedes and Ferrari that dominated the spotlight.
Gasly completed a full day of running in his Alpha Tauri and slowly eased himself into the session after trailing pacesetter Charles Leclerc by over five seconds in the early stages.
But by the end of the day, the Frenchman topped the timesheet with a lap of one minute 33.902secs, albeit on the softest on the five tyre compounds available.
It was Ferrari, however, that at this stage suggested it might have the edge, with Carlos Sainz and Charles Leclerc second and third quickest, both on the medium C3 rubber, with the Spaniard 0.457s adrift.
The session was brought to a premature close after Sergio Perez endured a clumsy spin in his Red Bull which resulted in him trundling backwards into a gravel trap with seven minutes remaining.
The Mexican finished the day 10th but posted the most laps of any driver with 138.
Mercedes could only manage P9 and P11 via George Russell and Lewis Hamilton respectively.
The team caused a stir, however, with its striking new sidepod design that Red Bull team principal Christian Horner has allegedly described as "illegal".
Haas missed the first four-hour session of the day due to a freight delay and will make the time up across the coming days.
Reserve driver Pietro Fittipaldi, however, was the slowest of all in his one outing before Kevin Magnussen returns to the cockpit.
Bahrain pre-season test day one times
1. Pierre Gasly [AlphaTauri] 1:33.902 
2. Carlos Sainz [Ferrari] 1:34.359 
3. Charles Leclerc [Ferrari] 1:34.531 
4. Lance Stroll [Aston Martin] 1:34.736 
5. Alex Albon [Williams] 1:35.070 
6. Lando Norris [McLaren] 1:35.356 
7. Valtteri Bottas [Alfa Romeo] 1:35.495 
8. Sebastian Vettel [Aston Martin] 1:35.706 
9. George Russell [Mercedes] 1:35.941 
10. Sergio Perez [Red Bull] 1:35.977 
11. Lewis Hamilton [Mercedes] 1:36.365 
12. Fernando Alonso [Alpine] 1:36.745 
13. Esteban Ocon [Alpine] 1:36.768 
14. Guanyu Zhou [Alfa Romeo] 1:37.164 
15. Pietro Fittipaldi [Haas] 1:37.422