Valtteri Bottas set the pace on Saturday in Bahrain, but the Finn played down the performance and highlighted his struggles with the "snappy and unforgiving" rear end of Mercedes' new W12.
Although he took to the track an hour into the afternoon session, Bottas enjoyed a clean run through the team's programs that contrasted with Mercedes' troubled Friday morning.
Bottas accumulated 58 laps and set the fastest lap so far in pre-season testing. The Finn was relieved but underscored the set-up work that remains to be done to iron out a few major issues impact Mercedes' W12, and specifically its rear end.
"I finally managed to get some laps and some clean runs to really learn about the new car and try some things," he said. ,
"It's always good when you learn and that's what testing is all about - the opportunity to run different things and see how the car reacts.
©Mercedes
"As a driver you use that information in a race weekend so it's hugely important. If the set-up changes work or don't work, it's all valuable information.
"For sure it's windier here than we've had at a race but one of the bigger issues with the car is the rear end, it's quite snappy and unforgiving.
"With the new tyres, it's quite sensitive so we need to calm the car down a bit."
Bottas' comments echoed those earlier in the day voiced by Lewis Hamilton who also singled out the W12's rear end which he said, "didn't feel great with this new regulation change".
Bottas was also reluctant to draw any conclusions from the performance that put him at the top of the time sheet on Saturday.
"It's so difficult to say what that means," he said. "I can't be jumping around happy - it is only testing.
"As always, we are just focused on getting through our programme and maximising the learning. You never know what fuel loads people are running.
"Hopefully by the end of tomorrow, we will have a better idea of where we are.
"Testing is limited and a lot more compromised this year for everyone, you have less time in the car before the first race so I would be happy if we could just get through the final day tomorrow nice and clean.
"Today felt good and the times were pretty fast, so I just want more laps and more feeling with car."
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