Valtteri Bottas feels Williams can still improve its package around the Baku City Circuit despite a strong start in the European Grand Prix saw him record strong lap times and a top speed of 351kmh.

The Finn was the closest rival to the Mercedes' in FP1 with the third fastest time before ending the day in fourth place as one of five Mercedes-powered cars to lock out the top five positions.

Spurred on by a mammoth top speed of 351kmh (218mph) on the lengthy home straight as Williams once more topped the speed charts, the Finn was impressed by the form of the FW38 but says good engine performance alone isn't necessarily critical to decent lap time.

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"We shouldn't underestimate the others and this track also has a lot of corners, so a car that is good in the corners can still be quick here," the Montreal podium winner said. "That's why we really need to improve for tomorrow because everyone else will.

"We still have a lot of work to do with the set-up of the car, I'm not entirely satisfied with the balance we have, with the handling, so we'll have to see tomorrow. I'm sure it's going to be a lot closer tomorrow than what we've seen today. The track will evolve a lot and both the team and the driver will have to keep up with that evolution

Suggesting speeds will rise further come qualifying, though Bottas admits his 351kmh pace was with a tow, he insists he isn't focused on chasing one when it matters.

"I did get a bit of a tow, but we are still to run the engine qualifying modes. It is a fast circuit, especially the corners leading to the main straight are pretty cool.

"A tow seems to make a big difference but it also ruin your lap in sector two, I've always preferred not to play with that too much, keep it simple and do the best lap time you can with the car you have."