Daniel Ricciardo and Red Bull laid down a marker for F1 2018 by posting the fastest time of pre-season testing on Day Two of the final test.
Ricciardo posted a best time of 1:18.047 on the hypersoft tyres to beat the previous benchmark for winter testing by nearly a second and set an unofficial track record for the Circuit de Catalunya.
The Australian finished 0.353s ahead of reigning world champion Lewis Hamilton, with Valtteri Bottas a further tenth back - both Mercedes drivers setting their best times on the ultrasoft tyres.
"I think if Mercedes put the hypersofts on they will be a couple of tenths ahead," Ricciardo said.
Ricciardo also completed the most individual laps of the day with 165, amounting to more than double a Spanish GP race distance.
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Day One pacesetters Ferrari were solid if not spectacular, with Sebastian Vettel fourth after a best lap of 1:19.541 and Kimi Raikkonen ninth with a 1:20.242.
But Ferrari opted against using Pirelli's quickest tyres, with Vettel setting his new personal best for winter testing on soft tyres.
"This is the day testing got serious," declared Sky F1's Ted Kravitz as the prospects of a three-way fight in F1 2018 increased.
"There are three quick teams and there's no doubt that Red Bull are going to be one of the teams we're going to be fighting with this year," Mercedes technical director James Allison told Sky Sports after seeing Ricciardo go fastest.
McLaren suffer yet more testing woe
With Wednesday bringing the best weather conditions of testing so far, nine of the 10 teams posted their quickest times of the winter.
The exception was McLaren who suffered their fourth breakdown in two days and were limited to just 57 laps.
Fernando Alonso had impressed early on, posting a winter personal best of 1:19.856 on hypersofts, but he stopped at Turn Seven after just two hours of running.
A "small" oil leak was identified as the cause but it required McLaren to change their new Renault power unit and it was not until the final 13 minutes that the Spaniard was able to return to the track.
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However, McLaren chiefs played down suggestions they are approaching another winter crisis with executive director Zak Brown insisting their problems have been minor "teething" issues.
"All the stuff that we've had have been pretty minor which get worked out at testing," he told Sky Sports News.
"We know what the issues are, they are all easily fixed. Of course, you would like to do more laps but we don't think we have any problems so we are not concerned."
Hartley and Toro Rosso best of rest
To rub salt into McLaren's wounds, Brendon Hartley surpassed the Woking team's best time of testing late in the day to put Toro Rosso fifth in the timesheet with a 1:19.823.
The Kiwi improved the team's best time of testing so far by 1.1 seconds, while his 119 laps was more encouraging reliability for the Honda engine.
Behind Alonso, compatriot Carlos Sainz was seventh fastest for Renault, with the French team also racking up the most laps of the day (190), Nico Hulkenberg contributing 102 in the afternoon.
Charles Leclerc was quietly impressive in the Sauber, posting a best time nearly a second up on team-mate Marcus Ericsson's quickest so far, although he brought out a late red flag when he slid into the gravel.
Esteban Ocon also passed a century of laps, although he was only 12th fastest in the Force India.
But Haas and Williams hit reliability problems, Romain Grosjean being limited to just 78 laps after an oil leak while Sergey Sirotkin's afternoon running was compromised when he came to a halt in the pit lane.