The big story was McLaren and Honda, which we will get to shortly.
Bottas set his best lap of 1m31.280s in the final hour as the sun went down and the asphalt cooled.
Vettel to set the second best time of 1m31.574s but his day was interrupted by several issues with the car.
Toro Rosso's Carlos Sainz Jr ended the day third fastest, but it was Stoffel Vandoorne and McLaren that had a really good day.
For the first time of the year McLaren and Honda had a trouble-free day of testing. Is it going to snow in the desert tonight?
McLaren racing director Eric Boullier called it the "best day" of testing that the team had enjoyed so far this year.
Vandoorne was able to set a lap of 1m32.108s – just eight tenths off Bottas's best lap of the day, and completed 81 laps – well over a race distance.
WEDNESDAY TEST TIMES:
|3||Carlos Sainz||Toro Rosso/Renault||1m31.884s||0.604s||68|
|6||Esteban Ocon||Force India/Mercedes||1m32.142s||0.862s||60|
|7||Daniil Kvyat||Toro Rosso/Renault||1m32.213s||0.933s||61|
|10||Pierre Gasly||Red Bull/Renault||1m32.568s||1.288s||65|
|12||Sergio Perez||Force India/Mercedes||1m35.015s||3.735s||70|