Valtteri Bottas comfortably clocked the fastest time of winter testing so far on the final morning of the first test, but Ferrari lost half the session after an engine problem struck their new car.
Bottas set a new benchmark of 1:15.732 with the eye-catching W11 - within three tenths of his lap record from last year's Spanish GP - in a short run on Pirelli's fastest tyres.
By this point, Ferrari were already sidelined in the garage replacing the engine in their SF1000 after a fault stopped their morning after 40 laps.
The first mechanical-related on-track stoppage of 2020 came as a blow to the Scuderia, with Vettel's first full day in the new car compromised as a result.
After experiencing their own delays on the opening two days, Renault enjoyed a more reliable morning and finished the session in second place with Esteban Ocon. Running slower tyres than Bottas, Ocon posted the RS20's quickest time so far and finished 1.370s adrift.
Racing Point's strong start to 2020 continued as Lance Stroll set the third-best time, just ahead of AlphaTauri's Daniil Kvyat.
Red Bull completed the most laps of any team in the opening four hours - 84 - despite the RB16 requiring repairs when Max Verstappen ran over the Turn Eight kerb.
"A good amount of laps, and especially for this first week that is the most important [thing]," said Verstappen to Sky Sports News.
"We just try everything we can to be competitive."
F1 Testing Timesheet: Day Three morning session
1. Valtteri Bottas, Mercedes, 65 laps, 1:15.732
2. Esteban Ocon, Renault, 76 laps, +1.370
3. Lance Stroll, Racing Point, 52 laps, +1.606
4. Daniil Kvyat, AlphaTauri, 62 laps, +1.895
5. Max Verstappen, Red Bull, 86 laps, +1.904
6. Antonio Giovinazzi, Alfa Romeo, 65 laps, +2.303
7. Carlos Sainz, McLaren, 76 laps, +2.542
8. Romain Grosjean, Haas, 48 laps, +2.648
9. Sebastian Vettel, Ferrari, 40 laps, +2.652
10. Nicholas Latifi, Williams, 44 laps, +3.272