McLaren drivers Lewis Hamilton and Jenson Button said they were expecting a difficult weekend at the European Grand Prix.
Button and Hamilton were 12th and 14th fastest in practice in Valencia on Friday and both admitted they were not happy with the handling of their cars.
Hamilton said: "It's going to be a tough one for us."
Button said: "On one lap, we're not great, but not too bad. On the long run, the time is just not there."
Both Britons were struggling with locking their front brakes on the long Valencia street track, which contains 25 slow- and medium-speed corners.
They were fourth and eighth in the first session, with Button ahead.
Hamilton, who is leading the championship by two points from Ferrari's Fernando Alonso, said: "I haven't been at all happy with the car, but you can't always get it right.
"This year we have been pretty good at getting the balance right throughout practice, and having nice smooth days, but today has not been one of those days.
"I think I know what the problem is and I'm confident overnight we'll rectify it."
Button added: "We're quicker than we look over one lap but the problem is you could get a lap out of it but you might not. It's on a knife-edge.
"It's very competitive. I feel good in the car. I'm confident we can find a set-up that is going to work for us but whether we will be quick enough to challenge right at the front I don't know."
Both men expressed their surprise at the pace of the Force India cars, whose drivers Nico Hulkenberg and Paul Di Resta were second and sixth fastest in the second session.
"Force India are extremely quick," Hamilton said. "We haven't seen that all year, and it seems like someone else is having their chance this weekend.
"That's a little bit of a surprise, they seem to have been the team who have been almost competitive but not quite there, but here they seem to be very competitive."
It has emerged on Friday that a plan to hold a London grand prix is among the bids being considered for the future of the Olympic Stadium.
Both men said they would welcome a second grand prix in Britain in addition to Silverstone, which has a contract with F1 until 2026 that includes an opt-out clause after 2019.
Hamilton said: "Silverstone, we've got to make sure we keep that, but I've always said having a race around London could be spectacular, it could be one of the best grands prix of the year. I'd definitely support it."
Button added: "You're only going to get positive comments when it comes to British grands prix.
"Twenty British grands prix would be amazing. Sorry to the rest of the world, but I'm very patriotic.
"We all love driving in front of a home crowd. It's a fantastic facility we're going to see open to the world in a few weeks. It would be great to have the opportunity to race there as well."
Even if the bid plan for the Olympic Stadium is accepted, a race in London would require a multi-million pound investment to source the sanctioning fee, which would be in the region of £20m a year, and build the track.
And there may not be space for a second race in the UK on an expanding and already crowded F1 calendar.