Although using the softest of the five compounds made available by Pirelli for testing with the C5, Bottas offered a hint of the speed potential of his Mercedes W11 with a lap of one minute 15.732secs at Barcelona's Circuit de Catalunya.
In context, the lap was half-a-second quicker than in testing at the track a year ago, and only three-tenths of a second behind the lap record he set with his pole position time of 1:15.406s in qualifying for last season's Spanish Grand Prix.
Bottas' nearest challenger was former Mercedes reserve Esteban Ocon, now with Renault, who finished 1.370s adrift, albeit using the soft C4 tyre, as was the case with the third-quickest in Racing Point's Lance Stroll, who was a further 0.236s down.
For four-time champion Vettel, following a strong start with 40 laps on the board in the opening 90 minutes, his morning was cut short by an engine issue within his SF1000 that is now due to be investigated in Maranello.
Vettel, who is not scheduled to run again until some point in the afternoon following the fitting of a new engine, was naturally down in a lowly ninth position of the 10 drivers, finishing 2.652s behind Bottas on the medium C3 compound.
The only driver behind Vettel on the timesheet was Williams' rookie Nicolas Latifi, who also brought out a red flag when his FW43 stopped towards the end of the main straight, with wisps of smoke emerging from the car, leaving him 3.272s adrift of Bottas.
Behind Stroll was the AlphaTauri of Daniil Kvyat, with his best lap set on the medium compound, almost 1.7s behind Bottas.
Red Bull's Max Verstappen finished with a morning-high 86 laps as the team opted for reliability running compared to pace, with the Dutchman's best time of 1:17.636s coming on the C2 hard rubber and good enough for fifth fastest.
Pre-season Testing Results – Thursday 20 February 1 Valtteri Bottas Mercedes-AMG Petronas 65 Laps 1:15.732 C5
2 Esteban Ocon Renault 76 Laps +1.370 C4
3 Lance Stroll BWT Racing Point 52 Laps +1.606 C4
4 Daniil Kvyat Scuderia AlphaTauri 62 Laps +1.695 C4
5 Max Verstappen Aston Martin Red Bull Racing 86 Laps +1.904 C2
6 Antonio Giovinazzi Alfa Romeo Racing ORLEN 65 Laps +2.303 C3
7 Carlos Sainz McLaren 76 Laps +2.542 C2
8 Romain Grosjean Haas 48 Laps +2.648 C3
9 Sebastian Vettel Scuderia Ferrari Mission Winnow 40 Laps +2.652 C3
10 Nicholas Latifi ROKiT Williams 44 Laps +3.272 C3
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