Mercedes driver Lewis Hamilton set an impressive pace on a damp final morning of the second pre-season test at the Circuit de Catalunya.
Hamilton was 0.351 seconds faster than second-placed Jenson Button, his former McLaren team-mate, as the track dried out after overnight rain in Barcelona.
Toro Rosso's Jean-Eric Vergne was third ahead of Sauber's Esteban Gutierrez and Jules Bianchi's Force India.
Bianchi is one of two hopefuls for the final available seat at Force India.
The other is German Adrian Sutil, who impressed on Thursday in his first run in an F1 car for 16 months.
The day started off damp, and the drivers initially went out on to the track on 'intermediate' tyres, but the threatened persistent rain did not materialise and by mid-morning drivers were able to start lapping on 'slick' dry-weather tyres.
The lap times quickly tumbled. Button set a one minute 27.679 seconds lap and lopped more than two seconds off it the next time around before Hamilton toppled him with an impressive 1:23.282.
Button then closed in on Hamilton with a 1:23.633 a few minutes later.
Hamilton's time was set at the start of a multi-lap run on the 'medium' tyres, which Button had also been using. Hamilton's times dropped off dramatically as he continued - within three laps of his fastest time he was more than four seconds slower.
The rain returned towards the end of the morning session, preventing any further improvements.
The drivers will be hoping for further dry running in the afternoon as they work to understand their new cars.
There are only four days of testing remaining before the start of the season in Melbourne, Australia next month.
The teams reconvene in Barcelona on Thursday next week, when most are expected to have on their cars the performance upgrades with which they will start the season.
SECOND PRE-SEASON TEST, DAY FOUR, CIRCUIT DE CATALUNYA, SPAIN, MORNING FASTEST TIMES
1. Lewis Hamilton (GB) Mercedes 1:23.282
2. Jenson Button (GB) McLaren-Mercedes 1:23.633
3. Jean-Eric Vergne (Fra) Toro Rosso-Ferrari 1:24.071
4. Esteban Gutierrez (Mex) Sauber-Ferrari 1:25.239
5. Jules Bianchi (Fra) Force India-Mercedes 1:25.732
6. Giedo van der Garde (Ned) Caterham-Renault 1:27.429
7. Felipe Massa (Brz) Ferrari 1:27.563
8. Mark Webber (Aus) Red Bull-Renault 1:27.616
9. Max Chilton (GB) Marussia-Cosworth 1:29.902
10. Romain Grosjean (Fra) Lotus-Renault 1:34.800
11. Valtteri Bottas (Fin) Williams-Renault no time