Finish driver Valtteri Bottas was fastest for Williams on the final morning of pre-season testing in Barcelona as McLaren's reliability problems continued.
Bottas bettered teammate Felipe Massa's time from Saturday on the supersoft tyres, but was still someway off Mercedes' best on the slower soft tyres in Barcelona in 1min 23.063sec.
Ferrari's Sebastian Vettel was second quickest when also running on the supersoft compound with Brazilian Felipe Nasr in third.
Nico Rosberg concentrated on mileage over speed for Mercedes with 69 laps down in fifth fastest.
However, McLaren's inability to put their MP4-30 through its paces ahead of the opening Grand Prix of the season in Melbourne in two weeks' time continued as Jenson Button could only muster two installation laps without registering a time before lunch.
Results from the morning session on day four on the third pre-season test in Barcelona:
1. Valtteri Bottas (FIN/Williams) 1:23.063 (45 laps)
2. Sebastian Vettel (GER/Ferrari) 1:23.469 (52 laps)
3. Felipe Nasr (BRA/Sauber) 1:24.023 (72 laps)
4. Sergio Perez (MEX/Force India) 1:25.113 (54 laps)
5. Nico Rosberg (GER/Mercedes) 1:25.186 (69 laps)
6. Max Verstappen (NED/Toro Rosso) 1:26.267 (49 laps)
7. Daniel Ricciardo (AUS/Red Bull) 1:26.587 (10 laps)
8. Pastor Maldonado (VEN/Lotus) 1:28.272 (35 laps)
9. Jenson Button (GBR/McLaren) no time (2 laps)