Ferrari and Sebastian Vettel showed Mercedes what they could be up against as the German edged out Valtteri Bottas on day two of testing in Barcelona.
The unpredictable weather in Catalonia suggested there could be plenty of excitement but both the Scuderia and Silver Arrows enjoyed relatively smooth days out on the track.
Vettel came in ahead of the pack, completing 98 laps of the Circuit de Catalunya for a total of 456km.
Rival Bottas managed four fewer but still managed to be just 0.303 seconds behind the leader, despite using a tyre one grade softer and slower.
World champion Lewis Hamilton was expected to make an appearance in the afternoon but Mercedes changed their plans in order to maximise his team-mate's running time on day two.
In terms of general operation it's been very smooth.Sebastian Vettel
The Brit is now likely to step out on Wednesday, once the team have made the necessary adjustments to their car to suite his driving style.
McLaren's Stoffel Vandoorne was third fastest, just ahead of Red Bull's Max Verstappen, while Carlos Sainz also produced an encouraging performance in his Renault.
The day, though, seemed to suggest it will be Ferrari who again pushes Mercedes hardest in 2018.
Vettel, who was one of three drivers to spin off the track, admitted the conditions in Barcelona made driving tough but said he was pleased with the overall performance of his SF71H.
"It's very cold. It's very difficult to get the tyres to work and I think everybody is having the same issues," said Vettel.
"The car is working, we didn't have any problems reliability-wise. In terms of performance, I don't know yet, but in terms of general operation it's been very smooth."
Vettel is 50/1 to win the Drivers' Championship, with Bottas 2000/1 and Hamilton available at 1/500.
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