Lewis Hamilton returned Mercedes to the top of the times while Red Bull's struggles continued during the final pre-season Formula 1 test of 2014 in Bahrain.
The 2008 world champion made a late start, after losing most of the morning to a gearbox issue on the W05, but worked his way to the top of the times as the afternoon progressed.
He fell just 0.02 seconds short of beating Felipe Massa's Sakhir circuit pre-season benchmark from Saturday's test and bumped Massa's Williams team-mate Valtteri Bottas from the top spot, after the Finn had led the way for most of the day before stopping on the start-finish straight with a mechanical issue.
Sebastian Vettel endured another frustrating day in Red Bull's RB10, suffering what the team described as "a mechanical issue at the front of the car", which pitched the world champion off at Turn 1 before lunch.
The German returned to the track in the afternoon, and racked up a total of 77 laps, but at reduced pace.
He wound up ninth fastest thanks to late improvements under the floodlights, but still over four seconds off Hamilton.
Fernando Alonso set Ferrari's fastest time of pre-season around the Sakhir circuit to go third fastest, after the F14 T caused one of eight red flags across the day when Alonso stopped at Turn 12.
Bahrain F1 testing gallery
Romain Grosjean's Lotus, Jenson Button's McLaren, and Nico Hulkenberg's Force India all suffered engine failures that curtailed their running early, while clutch issues forced Kamui Kobayashi's Caterham to stop prematurely too.
Jean-Eric Vergne enjoyed arguably Toro Rosso's most successful day of pre-season, clocking 74 laps and the fifth fastest time.
Adrian Sutil and Esteban Gutierrez split driving duties in the Sauber and racked up more laps between them (177) than anyone else.
Electrical dramas delayed Max Chilton, but the Brit made a late surge to seventh on the timesheet, splitting the Sauber drivers with Marussia's fastest lap of pre-season around this track.
Pos Driver Team/Car Time Gap Laps
1. Lewis Hamilton Mercedes 1m33.278s 70
2. Valtteri Bottas Williams-Mercedes 1m33.987s +0.709s 108
3. Fernando Alonso Ferrari 1m34.280s +1.002s 74
4. Nico Hulkenberg Force India-Mercedes 1m35.577s +2.299s 74
5. Jean-Eric Vergne Toro Rosso-Renault 1m35.701s +2.423s 74
6. Adrian Sutil Sauber-Ferrari 1m36.467s +3.189s 91
7. Max Chilton Marussia-Ferrari 1m36.835s +3.557s 61
8. Esteban Gutierrez Sauber-Ferrari 1m37.303s +4.025s 86
9. Sebastian Vettel Red Bull-Renault 1m37.468s +4.190s 77
10. Jenson Button McLaren-Mercedes 1m38.111s +4.833s 22
11. Kamui Kobayashi Caterham-Renault 1m38.391s +5.113s 106
12. Romain Grosjean Lotus-Renault 1m39.302s +6.024s 32