Felipe Massa was fastest, Daniel Ricciardo was close and Jules Bianchi had a small fire in an action-packed day one of testing at Silverstone.
Just a few days after Valtteri Bottas came from 14th on the grid to claim second place in the British GP, Williams continued to impress with Massa setting the fastest time.
The Brazilian posted a 1:35.242 in cooler conditions on medium tyres in the morning session and it was unrivalled for most of the day, but Ricciardo came very close to bumping him into second place in the last few minutes.
The Red Bull driver had spent much of the day running just sensors and not setting any lap times, but he gave it a go late in the afternoon and was rewarded with a P2 that was just 0.006s off the pace of Massa's FW36.
Nico Rosberg was there or thereabouts in third place with a 1:35.573 and his Mercedes was followed by Adrian Sutil in the Sauber and Bianchi for Marussia.
Bianchi, who will be testing for Ferrari on Wednesday, was behind the wheel of the MR03 on the opening day and he had an eventful session with a "small fire" causing a red flag and ending his session prematurely.
The Marussia driver also did some tyre testing for Pirelli along with Ferrari's Pedro de la Rosa, who was 10th fastest with only GP2 racer Will Stevens in the Caterham slower.
Jean-Eric Vergne was another man who caused a red flag as his Toro Rosso stopped on track, which led to a lengthy delay while the marshals cleared the track.
01 Felipe Massa Williams 1:35.242s 42 laps
02 Daniel Ricciardo Red Bull 1:35.248s +0.006s 72 laps
03 Nico Rosberg Mercedes 1:35.573s +0.331s 90 laps
04 Adrian Sutil Sauber 1:35.674s +0.432s 73 laps
05 Jules Bianchi Marussia 1:36.148s +0.906s 108 laps
06 Stoffel Vandoorne McLaren 1:36.462s +1.220s 71 laps
07 Sergio Perez Force India 1:36.583s +1.341s 79 laps
08 Jean-Eric Vergne Toro Rosso 1:36.688s +1.446s 28 laps
09 Pastor Maldonado Lotus 1:37.131s +1.889s 97 laps
10 Pedro de la Rosa Ferrari 1:37.988s +2.746s 49 laps
11 Will Stevens Caterham 1:40.627s +5.385s 95 laps
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