Sebastian Vettel was quickest on his Ferrari début but it was Mercedes leaving the bigger impression on the first day of pre-season testing in Jerez.
The German took over the top of the timings from Nico Rosberg early in the afternoon and continued to knock a few tenths off here and there eventually finishing with a 1:22.620, just under two tenths of the second ahead of Marcus Ericsson who completed a qualifying style run on the soft compound tyre in the Sauber.
Vettel also completed 60 laps on a productive day in the SF15-T with Ericsson managing 73 in the Ferrari-powered Sauber.
However, with lap times meaning very little for now, it was Mercedes' mammoth advantage in terms of distance covered that will give their rivals great food for thought looking ahead to the remaining days.
Rosberg completed 151 laps in all, more than two race distances on the first day which was more double than the next best driver setting the third quickest time with a 1:23.106.
McLaren-Honda off to a slow start
The day was also highlighted by the official return of Fernando Alonso to McLaren as the Honda-powered MP4-30 made its track début.
However the newness of the Japanese power unit compared to its rivals was shown as the Spaniard spent much of the day in the garage as the synchronisation process with all the systems meant it was of case of small issue after small issue, which is expected with a brand new car, rather than anything major.
As a result the McLaren brought up the rear of the field both in lap time and in laps recorded with a best of 1:40.738 from just six trips around the Spanish circuit.
Camouflaged Red Bull can't hide problems
Red Bull finally revealed their new car, the RB11, not that they wanted you looking too closely as they replaced the traditional blue, yellow and red livery with a black and white camouflage-style paint scheme.
Daniel Ricciardo enjoyed a much better first day than the team had a year ago at Jerez, where the RB10 completed just a handful of laps, but a few issues - seemingly to do with cooling - did restrict the Aussie to the second lowest total of the day with just 35 laps though he did set a more competitive lap time with a 1:23.338 to finish fourth.
Williams were one the later starters as they completed system checks in the garage but their recovery again highlighted the already impressive reliability of the Mercedes power unit with Valtteri Bottas completing 66 laps, just under a full race distance, and ending the day fifth in the standings.
Solid start for Sainz
Carlos Sainz Jr. had plenty of local support as he made his F1 début for Toro Rosso. The Formula Renault 3.5 champion's arrival has been largely overshadowed by his team-mate Max Verstappen but the Spaniard enjoyed a solid start to pre-season completing 46 laps and setting a 1:25.327.
There was one honour the Toro Rosso driver earned that it didn't want, however, as, on a day with excellent reliability out on the circuit, Sainz was the only driver to stop out on track causing a brief red flag period.
Missing from the day's action were Lotus as the new E23 was being flown from Enstone to the south of Spain and the team hope to begin their pre-season in earnest on Monday.
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