Jenson Button believes McLaren needs to improve a lot in order to feel confident of challenging Red Bull this weekend in the Japanese Grand Prix.
Although Button set the pace in both the morning and the afternoon sessions of practice on Friday, Red Bull's pace over long runs suggests the team is still favourite for victory this weekend.
Button was happy with his day's work, but conceded McLaren will need more speed to be able to match Red Bull.
"It's only Friday, but the car feels good around here," said Button. "I'm pretty happy with today but we need to improve a lot more to really feel confident that we can really challenge the Red Bulls.
"There are a few areas where we are still a little bit weak. We need to improve in terms of balance and hopefully we can find that overnight for tomorrow morning."
He added: "We don't [know what the real picture is]. I mean, they are extremely quick, but I don't think we are that slow. You never know what people are doing with fuel loads. Ferrari are possibly running with more fuel than they normally do because they had a shocking race in Singapore.
"We don't know what fuel people are running, so we just get on with our own work. We've made some good progress, but I think there are areas where we can improve."
Button admitted he is not thinking too much about delaying Sebastian Vettel's title celebrations, but rather about challenging for victory on Sunday.
The Briton is the only other driver with a mathematical chance in the championship, but he needs to win the race.
"He still has to not finish or finish outside the top ten, and he hasn't done that yet this year," said Button of Vettel.
"The important thing for us is to get our car sorted for the weekend and hopefully we will be quick enough to challenge for victory."