Silverstone: Jenson Button pinned his British Grand Prix hopes on McLaren working through the night on Friday after his revamped car proved more of a handful than expected.
The world championship leaders had hoped to see a big step up in free practice at Silverstone but, despite being cheered by a record-equalling 85,000-strong crowd, the evidence was far from comforting.
Lewis Hamilton, top of the overall standings after nine races, was second fastest in the morning session but fell back to eighth in the afternoon while world champion Button was eighth and 13th respectively.
Oscar out to prove oracle octopus wrong
"The car is very tricky in a few places around the circuit. We've definitely made improvements in certain places but I just don't think we have the balance right yet," said Button, currently second in the standings but yet to stand on his home podium in a decade in the sport.
"It's not terrible because we're not at the back but the Red Bulls are very fast and it's going to be tough to challenge them, so we just have to hope we make a good step forward tonight," he added.
Red Bull's Sebastian Vettel, winner last year at Silverstone as well as in Valencia two weeks ago, was quickest in the first session before Australian team mate Mark Webber set the pace in the afternoon.
"The good thing is we're very close to home, but we'll all be working very late tonight to get everything right for tomorrow," said Button.
Team principal Martin Whitmarsh told reporters that parts were still arriving at the circuit from the factory in Woking at 0130 local time on Friday.
"People have worked incredibly long hours, and day and night, to just get those bits here," he said. "The amount of effort people put in is phenomenal.
"The mechanics here worked very late and I'm sure they are going to be working through tonight as well."
Button took some comfort from the lesson of last season's Turkish Grand Prix, when he chalked up one of his most dominant wins after struggling on the Friday.
"It's hard to handle ahead of qualifying, which is a worry because that's an area where we have to be stronger," he said of the car. "At the moment we're not, but it's just a very different car to what we have driven before.
"We've done a lot of work, some positive, some not so positive, which is always the way when you are working with new parts.
"I think we can still do a good job but it's not going to be a walk in the park. The next 12 hours are going to be very busy," added the 30-year-old.
Powell promises fast time in 100 showdown with Gay
"In Turkey last year I really struggled on the Friday but won the race by over 30 seconds so it's not over yet, we've only had the first day of practice."
Hamilton, the 2008 champion whose style is more aggressive than Button's, was less concerned.
"The changes that we made to the car, set-up wise, actually didn't make it better," he told reporters. "But sometimes you find that's the case.
"So what we will do is go back to the set-up we had earlier on and start from there again. Hopefully tomorrow the set-up will be better to drive. Generally I think the performance of the car is good."