Valtteri Bottas set a new testing lap record for the Circuit de Catalunya as Mercedes ended the first in-season test on top of the timesheets in Barcelona.
Bottas posted a 1:16.904 on supersoft tyres in the morning session, lowering the previous record of 1:17.182 which Sebastian Vettel had set in pre-season testing.
The Finn also racked up 139 laps - more than double a Spanish GP race distance - as he finished 0.068s ahead of Antonio Giovinazzi's Ferrari, the Italian completing the most laps with 148.
Bottas' time was over six tenths quicker than the leading time Max Verstappen had set in the Red Bull on day one, but still well down on Lewis Hamilton's pole lap of 1:16.1 at the weekend's grand prix.
"Today was really interesting and we got through our programme fairly smoothly," said Bottas. "We had a good baseline from last weekend and used it to work on different set-ups, trying out bigger changes than we normally would on a race weekend.
"We also tried to gather some information on the tyres, as we will see similarly smooth tarmac at Silverstone and Paul Ricard."
British teenager Lando Norris recorded the third-fastest time of the day late on with a 1:18.039 as McLaren recovered from an on-track stoppage in the afternoon.
The Formula 2 championship leader's effort on soft tyres was nine tenths faster than Stoffel Vandoorne's best time on supersofts on Tuesday, although fuel levels and car settings are unknown.
"Today wasn't about testing while doing flat-out qualifying or race-spec driving as such, it was more about aero tests," said Norris. "It was also all about keeping it on the track, which I didn't quite do - a little spin into the entry of Turn 12, but nothing major - to be consistent and complete the tests for the team.
"It's good that I can show the team what I can do, but I see it more as an opportunity for me to keep learning and developing, and learn from both Stoffel and Fernando over race weekends, rather than proving that I can do a lap-time."
Norris' effort relegated Kevin Magnussen's Haas to fourth, while Force India's Nikita Mazepin ended fifth with a 1:18.344 lap in the final hour of the day.
British-Korean Jack Aitken had an encouraging debut in the Renault, completing 120 laps and ending sixth in the timesheet, just ahead of Suaber race driver Charles Leclerc.
Fresh from making his Grand Prix weekend return, Robert Kubica was back in the Williams FW41 and completed 123 laps, but the Pole was 2.3 seconds off Bottas' leading pace and down in eighth.
Pierre Gasly took over the Toro Rosso in the afternoon session from Sean Gelael and was ninth with a 1:19.410 effort on medium tyres, Gelael ending his running in 12th.
On his Red Bull debut, Jake Dennis was 10th after setting a best time of 1:20.440 in the final 90 minutes, the 22-year-old Briton managing 75 laps in total.
"Obviously it's a big step up from what I'm used to in GT racing but I got used to the speed and the grip quite quickly and we managed to get through the whole run plan in the end," he said.
"It was nice to change a few things to give me a proper representation of the difference between the simulator and real life. So we have a few things to work on back in Milton Keynes to help Max and Daniel [Ricciardo] and make it even more realistic for them. But overall, super happy, a big thanks to the team for giving me the opportunity, and hopefully I'll be back in the RB14 soon!"
With McLaren and Force India running two cars as they also conducted a Pirelli tyre test, Vandoorne was 10th fastest in the MCL33, while Nicholas Latifi was slowest of all as he carried out the role in the VJM11.
The final two days of in-season testing takes place at the Hungaroring following the Hungarian GP in late July and immediately before F1's summer break.