Felipe Massa put Williams on top of the times on the opening day of the final week of testing at Barcelona, as both Mercedes and McLaren had problems.
Massa's best lap of 1:23.500 put him well clear of his rivals, the Brazilian also having a very productive day with over 100 laps covered.
The quickest time was set with soft tyres.
Sauber driver Marcus Ericsson was second quickest after a late run with Pirelli's super-soft tyres, the Swede finishing over half a second behind Massa after more than 120 laps.
Morning pace-setter Lewis Hamilton finished the day in third after having to call it a day early because of a problem with one of his car's energy recovery systems.
Mercedes tried to fix the issue, but it was unable to and Hamilton ended up running only in the morning and covering 48 laps in what was one of the team's less productive days of pre-season testing.
Red Bull had a more useful day, with Daniil Kvyat covering over 70 laps on his way to the fourth fastest time ahead of the Lotus of Romain Grosjean.
Kimi Raikkonen had a low-key session in the Ferrari, but still managed 80 laps as the Italian squad continued to look strong in terms of reliability.
Toro Rosso's Carlos Sainz was seventh fastest after his day was hindered by an engine problem.
Jenson Button finished nearly eight seconds off the pace and at the bottom of the times after completing just seven laps before a hydraulic leak caused McLaren-Honda's day to come to an early finish.
Kvyat in the morning and Sainz later in the afternoon caused the two red flags of the day, the Spaniard stopping on track an hour into the afternoon session.
The Toro Rosso driver missed over two hours of running before he made it back to the track.
|4.||Daniil Kvyat||Red Bull-Renault||1:25.947||+2.447||75|
|7.||Carlos Sainz||Toro Rosso-Renault||1:27.261||+3.761||86|