Sebastian Vettel says that there is still work to do on SF15-T to ensure that it is competitive at the British Grand Prix, but he still expects Mercedes to be hard to beat.

Speaking after finishing third fastest in the afternoon practice session on Friday, the German confirmed that it had been a trouble-free day for his side of the garage, but that he still had to nail the ideal set-up for the tricky twists and flat-out blasts of Silverstone.

"I think we more or less got through everything we wanted to do, but we're not entirely happy yet with how the car feels," Vettel reported, "I am happy with [the sensation in the car], but I think we can improve. We tried a couple of things and now we need to find what was good, what was bad. We didn't have anything going wrong with the car, so it's all about finding a rhythm."

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Both Ferraris were just over three-tenths behind pacesetter Nico Rosberg, with Kimi Raikkonen shading Vettel with a late lap, but four-time world champion admitted that he remained doubtful that the margin could be maintained.

"I don't know what happened to [Mercedes] today," he said after Lewis Hamilton only managed fourth place in the afternoon session, "but we saw their pace in general and on the long run this morning, so they will be quick no matter what. Anything else would be a surprise, but if was can repeat [this performance] tomorrow it will be good."