Valtteri Bottas demonstrated his readiness to step into the shoes of retired world champion Nico Rosberg by setting the pace, but Ferrari showed further signs that they could challenge Mercedes this season on day three of pre-season testing in Barcelona.
The limits of more powerful, faster cars allowed this year by radical rules changes were pushed as Bottas set the fastest time at the Circuit de Catalunya since 2009 in 1min 19.705sec on supersoft tyres.
However, four-time world champion Sebastian Vettel's second fastest time of 1:19.952 on soft tyres was quicker than Bottas on the same compound.
Flying Finn Bottas handed over to Lewis Hamilton for the afternoon session, but the Briton didn't trouble the top of the timesheets as he focused on race simulation and more miles in the Mercedes W08.
Hamilton completed 95 laps in just a few hours in the seventh fastest time.
"I think it was another good session. We completed lots of mileage and the entire programme that we'd planned to do -- including some long and short runs, collecting information on the different tyre compounds," said Bottas.
"We did some aero work to start with in the morning when the conditions were cold, then moved onto testing different settings on the car. It was a pretty straightforward day again, so I think everyone has done a great job so far. The car is running well and it's feeling better and better."
Red Bull's Daniel Ricciardo claimed on Tuesday that Mercedes can be beaten this season after three years of dominance.
And the Australian and Dutch teammate Max Verstappen look to have at least the third best car on the grid as Ricciardo posted the third best time of the day in his 70 laps.
"I definitely learned more about the car and the tyres today and really started to feel the downforce," said Ricciardo.
"I think when we start doing 60 laps in the heat you're going to see some physical limits being pushed for us drivers."
There were promising signs for Renault as Jolyon Palmer and Nico Hulkenberg registered the fourth and fifth fastest times.
However, McLaren-Honda's troubles continued as two-time world champion Fernando Alonso only managed the 10th fastest time despite trying out ultrasoft tyres.