The British team now lead the Formula One Constructors' Championship following Valtteri Bottas' maiden win in Russian Federation last time out and they topped their first practice session since the opening race in Australia.
Mercedes' main rival Ferrari was again adrift, albeit closer than it had been in the morning session, Kimi Raikkonen leading the team's effort with a time three tenths off the pace.
Red Bull and their driver Max Verstappen have fond memories of Barcelona as the Dutch teenager scored his first and so far only career win there a year ago, in his first race for them after joining from sister team Toro Rosso.
"And now it's done all I want to do is to go and win again and again", said Bottas.
In a session where many drivers were struggling for grip and balance, there were off-track excursions for the likes of Bottas, Vettel, Max Verstappen, Kevin Magnussen and Carlos Sainz.
After three races in 2017, some were saying the Finn was teammate Lewis Hamilton's mere "number 2".
And asked if he agreed with the assessment of former Ferrari driver Felipe Massa that the Scuderia would throw the bulk of their support behind Vettel, Hamilton told Sky Sports News HQ: "I think they are always like that".
Although most teams were expected to bring significant upgrade packages to Barcelona, Mercedes still caused a surprise by revealing several radical differences to the front of the W08.
Formula 1 returns to Spain this weekend and Lewis Hamilton will be hoping the change of scenery prompts a change of fortune as he looks to get back on track following recent struggles.
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In Russia previous year Ferrari was 0.7 seconds off pole, but it fell to 1.1 seconds behind in Barcelona.
"Really developing the auto, making it move forward to try and win this championship".
"It is definitely a step forward", Hamilton said of the upgrade. Marc Gene, the former Buckingham University student, who is the team's test and reserve driver and brand ambassador, said: "We are all keeping our fingers crossed for another good week for the team. There will be upgrades for everybody, and we will be doing work tomorrow to get the auto in the right place".
"We've since been unpicking what happened to understand why we couldn't get the vehicle in the right window so he could feel comfortable with it". In the last race, it did not start well and did not get any better.
"It makes us try harder".
Sebastian Vettel refused to deny he has had talks with Mercedes over a move to join Formula One's world championship-winning team in 2018.
Mercedes have brought major upgrades to their vehicle this weekend as they aim to stay ahead of the Ferrari's.
Qualifying on Saturday is at 2pm and the race on Sunday starts at 2pm.
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