Valtteri Bottas edged out Sebastian Vettel on the final day of the Bahrain in-season Formula 1 test on Wednesday, and clocked up an impressive 143 laps.
Bottas spent his day focused on both aerodynamic work and better understanding of tyres, with Mercedes having made clear it was trying to crack its recent struggle with long-run form this week.
Despite causing one red flag by stopping on track due to a mechanical problem just before lunch, Bottas got through an intensive programme and set his best lap of 1m31.280s in the final hour as track conditions cooled.
His lap tally was in stark contrast to Ferrari, as Vettel endured frustrations with a large amount of time lost in the pits.
In the morning, Ferrari discovered some hydraulic leaks during a set-up change in the garage and the work required left him on a tally of just eight laps before lunch.
After getting going again shortly after 1pm, Vettel lost further time when data problems in the garage left engineers without telemetry and he had to stay put until the issues could be rectified.
In the end, Vettel managed to get more more running done later in the afternoon to set the second best time of 1m31.574s.
Toro Rosso's Carlos Sainz Jr ended the day third fastest, having run only in the afternoon, while Stoffel Vandoorne finally gave McLaren something to smile about with the fourth fastest time with a late lap in the closing minutes.
Despite no changes having been made to the engine specification since Tuesday's problems, McLaren endured a pretty much trouble-free day of testing and Honda encountered no repeat of the MGU-H problems that have dogged it for the past week.
In fact, McLaren racing director Eric Boullier called it the "best day" of testing that the team had enjoyed so far this year.
With a late short run, Vandoorne was able to set a lap of 1m32.108s - just eight tenths off Bottas's best lap of the day.
More encouraging, though, was that Vandoorne completed 81 laps - more than a race distance.
Kevin Magnussen continued work on Haas's evaluation of brakes to set the fifth fastest time, while Esteban Ocon's morning time in the Force India was good enough to leave him sixth overall at the end, ahead of Toro Rosso's Daniil Kvyat and Williams driver driver Gary Paffett.
Renault's Sergey Sirotkin finished his first day of 2017 testing with the French car manufacturer ahead of Red Bull's Pierre Gasly and Sauber's Pascal Wehrlein.
WEDNESDAY TEST TIMES: