After "a good few weeks on track", Nico Rosberg has urged Mercedes to continue pushing as there is always more performance to gain.
Having finished runner-up in Mercedes' second 1-2 result of the Championship on Sunday night, Rosberg returned to the Sakhir circuit on Tuesday for Day One of the in-season test.
The German, who worked through set-up evaluations, covered 120 laps with a best time of 1:35.897 to finish P1.
But despite yet another pace-setting lap time for a Mercedes driver, Rosberg reckons there is still more to come from his team and their W05.
He said: "Our main aim was to try different directions with set-up and to get to know the tyres a bit better.
"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.
"We've had a good few weeks on track but you are never at the maximum level of performance.
"There is always more to be found and we are constantly pushing to improve the car and also ourselves as drivers.
"Having said that, the team are doing a fantastic job with this car from every perspective. Reliability, car performance, Power Unit performance: it's all there and it's a pleasure to be a part of a team with such positive momentum."
Gallery: Sensor Centre In Sakhir