Red Bull's hopes of getting their 2014 preparations back on track with a trouble-free final test evaporated on the first day's running in Bahrain.
Daniel Ricciardo managed his team's fastest time at Bahrain this winter early on, but was consigned to the garage for most of the rest of the day.
Ricciardo went back out late on but finished seventh, 2.6 seconds adrift of Force India's pace-setter Sergio Perez.
Williams's Valtteri Bottas was second fastest, then Ferrari's Kimi Raikkonen.
Nico Rosberg of pre-season favourites Mercedes was fourth quickest, after the team ended their running a little under an hour early following a problem the team declined to identify.
Testing times are notoriously unreliable indicators of real form as it is impossible to know the specification in which the teams are running their cars.
Predicting relative pace is even more difficult this year following the introduction of a fuel limit on the new hybrid turbo engines that will be used for the first time this season.
Nevertheless, insiders are unanimous that Mercedes are in the best shape ahead of the new season, and Thursday's running did little to change that impression.
Rosberg, who hands over to team-mate Lewis Hamilton for Friday, completed 89 laps.
The German was 1.334 seconds slower than Perez's fastest lap of one minute 35.290 seconds, but the Mexican used softer - and therefore faster - tyres to set his time.
Rosberg's problems meant all of last year's three leading teams hit trouble.
Raikkonen's pace came despite a troubled day which featured a long spell in the garage in the morning while the Italian team fixed an electrical problem. The car also stopped on track right at the end of the session.
Ferrari introduced a new front wing for the Finn to try, the first of what are expected to be several major upgrades with the first race of the season in Australia on 16 March in mind.
Raikkonen's time was Ferrari's fastest at Bahrain this winter - just under 0.1secs quicker than team-mate Fernando Alonso managed last week. The Spaniard takes the car over on Friday.
Ricciardo managed 32 laps early in the day but then the Red Bull spent five hours in the garage while the team worked to solve what were reported to be overheating problems - one of the recurring difficulties they have been experiencing this winter.
They managed to fix the car sufficiently for Ricciardo to get out on track again in the last half-hour of running, but he did only three laps before returning to the garage.
A further foray just before the end of running took his total to 39 for the day. That means Red Bull have in nine days so far done just 176 laps over their entire pre-season testing programme.
Renault, which supplies Red Bull, Lotus, Toro Rosso and Caterham, have had a troubled pre-season, at the root of most of the problems suffered by their teams.
Their teams continued to struggle at the start of the final test.
Lotus managed to 31 laps, and 10th quickest time, before the team had to stop running more than an hour early to sort out "exhaust and assorted changes".
Toro Rosso managed 56 laps with Daniil Kvyat but Kamui Kobayashi's Caterham could do only 19 in the entire day.