Lewis Hamilton recovered from an early setback to post the fastest time on the opening day of Formula 1's first in-season test at Bahrain.
The Mercedes driver caused an early red flag when his car ground to a halt just 15 minutes into the hit out due to a loose electrical connection on his W08.
Once rectified Hamilton completed a series of long and short runs which resulted in a session best 1:31.358s, set on soft compound tyres.
The session was initially delayed by 10 minutes to allow track officials to move shipping containers away from the pit lane exit.
Hamilton's time proved 0.626s faster than Ferrari's Antonio Giovinazzi who was tasked with the bulk of the development work for the Maranello team.
Regular driver Sebastian Vettel was enlisted to carry out 2018 Pirelli tyre development for the Italian tyre supplier.
A engine issue halted Vettel's progress although Ferrari was granted extra testing time given its Pirelli commitments.
Red Bull ended the day third fastest with Daniel Ricciardo lapping within a second of the Mercedes.
The Australian focussed much of his day working gathering aero data before working on tyre performance.
While the session was productive, a Renault engine failure cut short his track time.
The Milton Keynes squad will replace the power unit ahead of Pierre Gasly's stint in the RB13 today.
Romain Grosjean was fourth quickest as Haas F1 tested trailed new Carbone Industrie brakes.
Formula 2 driver Sean Gelael endured an eventful stint in the Toro Rosso which was interrupted by an electrical issue before causing a red flag in the afternoon.
Marcus Ericsson logged the most mileage by recording 106 laps behind the wheel of the Sauber.
Meanwhile, McLaren's problems continued as the Woking squad was restricted to just 17 laps after incurring another Honda engine failure.
Development driver Oliver Turvey was able to run in the final hour with his best time 3.6s adrift of the pace.
The final day of the test will begin later today.
Result: Formula 1 in-season test Bahrain Day 1