Valtteri ended a difficult session almost seven tenths of a second back from Lewis, after struggling once again with setup.
"So far, it's been a difficult weekend for us," said Valtteri. "We're clearly third quickest around here, that's just not enough.
"We've been chasing the setup of the car and made big changes yesterday and today, but we still haven't found the optimal setup. You can see it in the lap times that I can't really drive the car with the setup that we've got.
"We were a bit surprised by how big the gaps to Red Bull and Ferrari are. Two weeks ago, we had a great weekend in Monza and now we are really struggling.
"The result today highlights that we still have issues producing enough downforce and mechanical grip on a track like this; it's an area we clearly need to work on.
"We know our pace alone isn't enough to score a podium tomorrow, so we need a clean race, a good strategy and bit of luck as well."