The Finnish driver lapped the desert track an astonishing 143 times on a mammoth day of running for Mercedes, clocking a best time of 1m 31.280s in cooler conditions towards the end of the session.
Bahrain winner Sebastian Vettel was second fastest as he resumed normal Ferrari duties following a day of Pirelli tyre testing on Tuesday, though it was far from a trouble-free session for the Prancing Horse.
Mechanical issues meant that Vettel was able to complete just eight laps before lunch, with further issues in the afternoon limiting the German to a total of 64 for the day.
Toro Rosso’s Carlos Sainz was the only other driver to lap in under 1m 32s having taken over driving duties from team mate Daniil Kvyat, who finished P7, at lunchtime.
After logging minimal mileage with Oliver Turvey at the wheel on day one, Stoffel Vandoorne offered McLaren some encouragement as he clocked 81 laps - the sixth highest tally of the day.
The Belgian, who wasn’t able to start Sunday’s race because of a power unit issue, led a trio of closely matched drivers as he took fourth place on the timesheet, just ahead of Haas’s Kevin Magnussen and Force India’s Esteban Ocon, who shared cockpit time with team mate Sergio Perez.
Former McLaren test driver Gary Paffett was the only driver other than Bottas to log more than 100 laps as he completed his first day of running in Williams’ FW40, while fellow tester Sergey Sirotkin racked up 90 laps for Renault as he finished just behind the veteran Briton in ninth.
The timesheet was rounded out by Red Bull youngster Pierre Gasly, Sauber’s Pascal Wehrlein and the aforementioned Perez.
Unofficial Wednesday times from Bahrain
|3||Carlos Sainz||Toro Rosso||1:31.884||0.604||68|
|6||Esteban Ocon||Force India||1:32.142||0.862||60|
|7||Daniil Kvyat||Toro Rosso||1:32.213||0.933||61|
|10||Pierre Gasly||Red Bull||1:32.568||1.288||65|
|12||Sergio Perez||Force India||1:35.015||3.735||70|