Valtteri Bottas set the fastest time on the second day of pre-season testing in Bahrain on Saturday while world champion teammate Lewis Hamilton said there "was no point being worried" despite spinning off the dusty Sakhir circuit.
Bottas clocked 1min 30.289sec off 58 laps, 24 hours after finishing the opening day at the bottom of the time charts due to a gearbox problem that restricted him to just six laps.
Seven-time world champion Hamilton, who was down in 10th place on Friday, was 15th on Saturday.
Only four-time champion Sebastian Vettel, in his first season with Aston Martin after leaving Ferrari, was slower.
Hamilton suffered a rare spin in the morning session before Bottas took over driving duties under the lights in the afternoon.
"It's very gusty, as I found out into Turn 13. The rear doesn't feel particularly great but we are trying to find the sweet spot," said the Briton.
"It's day two of testing so we're just focused on understanding the car so there is no point being worried just yet."
Pierre Gasly, in an AlphaTauri, was second fastest, ahead of McLaren's Lance Stroll.