Lewis Hamilton says "dominant" Red Bull will make victory difficult in Sunday's British Grand Prix, despite qualifying on pole by nearly half a second.
Hamilton and team-mate Nico Rosberg completed an all-Mercedes front row ahead of the Red Bulls of Sebastian Vettel and Mark Webber.
"Red Bull have been dominant throughout the year," Hamilton said.
"We are getting closer but I don't feel we are there yet. We'll do what we can. We're in the best position to fight."
Mercedes have struggled with heavy tyre usage in race conditions so far this year and Hamilton admitted that was still a concern.
But he said he was hopeful that some promising race-simulation runs in Friday practice were a good omen.
"Undoubtedly it's going to be tough to keep Sebastian behind us," Hamilton said, "but our long-run pace wasn't as bad as we've seen in the past, so I'm hoping with the temperatures and a bit of care we can nurture the tyres to get a good result.
"I'm going to be pushing, giving my all, as I'm sure Nico will, to finish up ahead."
"We're not even certain we can stay ahead of the Red Bulls so what's the point of even thinking about [winning]?" he said. "We're just going to put as much positive energy as we can towards it and we'll do the best job we can."
Silverstone is a similarly demanding track on the cars to Spain's Circuit de Catalunya, where Rosberg and Hamilton fell to sixth and 12th from the front row of the grid.
But Rosberg said he felt the team would be in better shape at Silverstone.
"It's not difficult to do better than the Spanish Grand prix so we are definitely looking better than that," he said.
"We have made changes to the car all along, trying to understand it better but it's very complex," he said. "But for sure there are other teams doing a better job on Sundays, notably probably the Red Bulls who are just behind us.
"But we had decent runs on Friday with the high fuel so it should be possible to do a good result but how good I don't know yet."
Brawn said he was "not confident" the team had fixed their tyre problems.
"We have made a lot a modifications to the car and we'll see the effect of those modifications," Brawn said.
"You don't know until you get into a race how effective they've been. We need to take a step back and see how we can approach the car differently. We have some different things on the car this weekend and let's see how they work out."