Nico Rosberg topped the first day of the first in-season test for Mercedes in Bahrain.
Rosberg posted a time of 1:35.697 to top the timesheets and also completed the most laps, with 121.
"Our main aim today was to try different directions with set-up and to get to know the tyres a bit better," he said.
"You never stop learning how to get the best out of the tyres, so it has been a productive day from that perspective. We also did a bit of work on the brakes as this is an area I'm not one hundred percent happy with at the moment."
Force India's Nico Hulkenberg was second, head of Ferrari's Fernando Alonso.
Kevin Magnussen was a late addition to the test line-up for McLaren and he was fourth quickest in the MP4-29.
Valtteri Bottas was fifth for Williams, ahead of Marussia's Max Chilton, whose running was briefly disrupted by a hydraulics issue.
Daniel Ricciardo finished in seventh for Red Bull, ahead of Sauber debutant Sergey Sirotkin.
"Everything felt great today," said the Russian. "It was the first time for me in Bahrain and I liked the track. In comparison to what I am used to driving in World Series by Renault, I have quite a lot more engine power and more acceleration with the Sauber C33. On my fast lap in the morning I made a few mistakes, so I probably lost a second.
"But all in all, it was a good start to the session. In the afternoon we had some difficulties with my shoes. They were too small, so it was pretty painful when I was braking. When we changed the shoes, it was still not the best and difficult to drive. It was not easy, but I am glad we completed the 300 kms."
Robin Frijns was ninth for Caterham as they focused on tyre testing for Pirelli, while two power unit related failures restricted Pastor Maldonado to just 16 laps. Daniil Kvyat rounded out the runners.