Nico Rosberg topped the times for Mercedes in Friday's second free practice session ahead of this weekend's British Grand Prix - and then warned he may lack the race pace to win Sunday's showdown.
The German, who celebrated his 28th birthday on Thursday, clocked a best lap of one minute and 32.248 seconds in improving drying conditions after a rainswept morning.
That time was enough to lift the Monaco Grand Prix victor almost three-tenths of a second clear of nearest rival Mark Webber of Red Bull.
"The afternoon went well," Rosberg said.
"On one lap I think the car is still quick, as we have seen in the last few months, so qualifying should be OK.
"Then the only question mark is our race speed. We practised for the race speed today. It seemed to be OK, decent, but I'm sure it's still going to be a little bit of a problem for us.
"The next thing is that it's going to be another 10 degrees more tomorrow and Sunday and that could completely change the picture again.
"That will probably make it a little bit more difficult for us so we will just have to wait and see and make the most of it."
While Rosberg reflected on his chances of securing a second win this season, Australian Webber showed all the grit and commitment that has earned him four successive podium finishes at Silverstone and put him within reach of a hat-trick of British wins in five years.
After making public on Thursday his decision to leave Formula One and move to sports car racing with Porsche next year, Webber appeared to drive with great freedom and purpose on one of his favourite tracks.
His Red Bull team-mate defending triple world champion Sebastian Vettel, who only learned of his team-mate's decision after arriving at the track on Thursday, was third-quickest ahead of Britons Paul Di Resta of Force India and Lewis Hamilton in the second Mercedes.
Hamilton remained a hot home favourite to fight for pole and victory this weekend despite Mercedes' problems with balance and set-up.
Daniel Ricciardo, tipped to be a contender for Webber's seat in 2014, was sixth in the leading Toro Rosso ahead of his team-mate Jean-Eric Vergne, another Red Bull possible.
Adrian Sutil was eighth in the second Force India ahead of Romain Grosjean in the leading Lotus.
Two-time champion Fernando Alonso of Ferrari was down in 10th place while his team-mate Felipe Massa wound up 22nd and slowest after suffering a crash only 25 minutes into the 90-minutes session.
Jenson Button, the 2009 world champion who has never finished better than fourth in his home race in 13 years, was 11th for McLaren, two places better than 2007 champion Kimi Raikkonen of Lotus, tipped as favourite to succeed Webber alongside Vettel next year.
After a cold and rainswept morning, which saw few cars clocking timed laps in opening practice, the afternoon was warmer and mostly drier with the air temperature hovering around 18 degrees.
The opening period saw most cars testing the conditions with intermediate wet tyres before switching over to mediums as a dry line emerged and encouraged faster times.
Massa, who has endured major crashes at the last two Grands Prix in Monaco and Montreal, hit the barriers again when he lost control of his Ferrari in the exit from Stowe corner.
The crash left his car damaged and ruled him out of the rest of the session which, on a day when the team did little or no running in the morning, left Ferrari short of mileage.
At that time, Webber was the fastest man on track in his Red Bull before Rosberg set an early fastest lap around the 5.891-kms circuit in one minute and 32.680 seconds.
Vettel dislodged his compatriot but was soon replaced himself as Rosberg made the most of the improving conditions to go quickest again in 1:32.248 and stay there.