Alfa Romeo newcomer Kimi Raikkonen was sixth fastest, a tenth behind Vettel, and just ahead of the two Renaults of Nico Hulkenberg and Daniel Ricciardo.
"I have a positive buzz from driving the auto today, this is what I love doing".
By any measure, Williams have had a chaotic preparation for the season-opener, with technical director Paddy Lowe taking "a leave of absence" after they failed to finish their new auto in time for the start of testing in Barcelona and turned up two days late.
"We've got a contract that says that we're the first race and that runs until 2023. We've been trying to plan with the team all my sponsor events a bit more in advance so I can plan my calendar a bit more around it, all the details in terms of travel, logistics, hotels, everything to help my wellbeing and performance, same thing with the training we try again to improve from past year".
Hamilton is gunning for a third world crown in a row to close in on Michael Schumacher's all-time record of seven.
With Ferrari slightly quicker than Mercedes in winter testing, pundits were quick to question whether the "Scuderia" were "sandbagging" - keeping their true pace in reserve ahead of Saturday's qualifying.
"The auto felt better than it did at testing", said Bottas.
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"We didn't bring a lot of new parts to the vehicle, but we were able to optimise the package that we have", he said. It's how Formula One cars are supposed to be when they're wide and they're big. Feels quite odd. Seems to be something wrong.
A red flag was thrown, despite Albon being able to recover to the pits missing his front wing but with seemingly minor damage to the rest of the auto.
Charles Leclerc followed in ninth in the second Ferrari after spinning his vehicle late in the afternoon session, and Haas driver Romain Grosjean rounded out the top 10.
"It has been alright".
"We have a great pairing and the contribution from Valtteri and I together works, it's worked well for years and there's no reason to change it", he said.
The session was suspended for four minutes after Alexander Albon spun at Turn 2 and lost the forward wing on his Toro Rosso.
In between the practice sessions, FIA President Jean Todt and Formula 1 chairman Chase Carey attended a news conference which started with a minute of silence in memory of F1 race director Charlie Whiting, who died in Melbourne early Thursday from a pulmonary embolism.