Jenson Button's pre-season got off to an almost perfect as the McLaren driver set the pace at Jerez despite his early morning stoppage.
As the Formula One teams pitted their 2013 cars against one another at Jerez, the action began with 11 cars out on track but ended with just ten as Mercedes were unable to recover from their early woes.
Mercedes called it quits around midday after an electrical problem - the result of a faulty wiring loom - sparked a fire in Nico Rosberg's car during the morning's session.
The German, who had covered just 11 laps with a best time of 1:20.846, was forced to stop out on track as the flames licked the right rear of his new W04.
Rosberg's car was returned to the pits on the back of a flat-bed truck, however, hours later Mercedes revealed they would not be continuing for the day.
"After identifying the cause of the problem we will not be running again today whilst parts are modified ready for tomorrow," said the team via Twitter.
"The part which requires modification is the wiring loom where the electrical fault originated this morning."
Rosberg was joined on the sidelines for several hours by Day One's pace-setter Button who brought out the day's first red flag when he broke down after just three laps.
It emerged that a fuel pump-related issue was the cause of the McLaren driver's stoppage. "Not a good start," said team boss Martin Whitmarsh.
But by the end of the day it was a great start for McLaren as Button, who recovered to complete 37 laps, set the pace with a 1:18.861.
That put the 2009 World Champion 0.848s up on Mark Webber, who had the honour of being the first to run the new Red Bull racer, the RB9.
Romain Grosjean, who had a late start to the morning but had been leading by midday, finished third quickest but still within a second of Button's best.
Paul di Resta in his Force India VJM06 had a promising start was he was fourth quickest ahead of Daniel Ricciardo and Felipe Massa.
Meanwhile, Nico Hulkenberg made his Sauber bow, finishing seventh quickest, while Giedo van der Garde was in action for Caterham and Max Chilton for Marussia. They finished three and five seconds off the pace respectively.
Unofficial Times
01 Button McLaren 1:18.861 37 laps
02 Webber Red Bull 1:19.709 +0.848 73 laps
03 Grosjean Lotus 1:19.796 +0.935 54 laps
04 di Resta Force India 1:20.343 +1.482 89 laps
05 Ricciardo Toro Rosso 1:20.401 +1.540 70 laps
06 Massa Ferrari 1:20.536 +1.675 64 laps
07 Hulkenberg Sauber 1:20.699 +1.838 79 laps
08 Rosberg Mercedes 1:20.846 +1.985 11 laps
09 Maldonado Williams 1:20.864 +2.003 84 laps
10 vd Garde Caterham 1:21.915 +3.054 64 laps
11 Chilton Marussia 1:24.176 +5.315 29 laps
