Valtteri Bottas edged out Lewis Hamilton in opening practice for the British GP as Mercedes stole a march on their rivals at Silverstone.
Fresh from his impressive second career victory last Sunday in Austria, Bottas was fractionally the faster of the two Mercedes drivers in Practice One with the W08 immediately lapping at outright lap record pace around the famous old Northamptonshire circuit.
Bottas's 1:29.106 was just 0.078s quicker than Hamilton on soft tyres, but championship rivals Ferrari were a full second adrift despite running the grippier supersoft tyres.
Championship leader Vettel started his session by completing the first public test of 'The Shield' cockpit protection device on his installation lap, before reverting to his car's normal specification for the rest of the session.
But that did not stop the German, who was sixth behind team-mate Kimi Raikkonen, spinning at Maggotts and then running wide over the Turn Three kerbs.
Both Mercedes and Ferrari are introducing their third engines of the season this weekend.
Red Bull, running more upgrades on their RB13, split Mercedes and Ferrari with Max Verstappen, desperate for a race finish this weekend after five DNFs in the last seven races, within half a second of the Mercedes' in third.
"We can improve but we're not a million miles off," Daniel Ricciardo, who was fourth, told Sky F1.
"Mercedes look really strong but we look to be in the fight with Ferrari. At the moment we're battling Ferrari and not Mercedes."
Fernando Alonso was an encouraging eighth with both McLarens in the top 10, but the Spaniard take at least a five-place grid penalty on Sunday for again exceeding engine component limits.
Williams' Felipe Massa was the only driver in the top 10 other than the two Mercedes' to set his quickest time on the softs, the slowest tyre available this weekend.
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