Following his early morning issues, Lewis Hamilton wrestled back the fastest lap at the end of the first day of in-season testing at Bahrain.
The Mercedes driver fell victim to an electrical issue early on but once the problem was solved, the Brit ran through his assigned workload as scheduled.
Alternating short and long runs as Mercedes put its tyres through its paces in a bid to understand their weakness in Sunday's race, Hamilton ended the day with 1m31.258 run on soft rubber.
Sebastian Vettel focused on development work for Pirelli, leaving Ferrari resreve driver Antonio Giovinazzi to handle the team's development programme.
The German driver's day was partly compromised however when an engine problem set him back, but Pirelli received authorization to prolong its session by three full hours.
Daniel Ricciardo, the morning's pace setter settled for third but his day was also cut short when an engine failure seized his Red Bull RB13 which was left stranded at Turn 1.
The team will fit a replacement unit for Max Verstappen's day tomorrow.
Romain Grosjean sampled new brakes from Carbon Industrie as the team's mulls a supplier change. The Haas driver ended the day fourth, just in front of Williams' Felipe Massa and Renault's Nico Hulkenberg.
McLaren's Oliver Turvey, who completed just two installation laps in the morning before a leak was discovered with his Honda's ERS, took to the track late in the day, completing 17 laps and ending the day the bottom of the timesheet.
GALLERY: All the action from the Day 1 in-season testing at Bahrain
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