SILVERSTONE, England (Reuters) - Lewis Hamilton recognised he had work to do after Ferrari's Kimi Raikkonen and Sebastian Vettel lapped faster than the Formula One world champion in British Grand Prix practice on Friday.

"Ferrari look strong again here and they were right there with us today so it should be a good battle," the Mercedes driver said after finishing fourth in the day's second session led by team-mate Nico Rosberg.

Raikkonen was second and four-time champion Vettel, the only non-Mercedes driver to win this year, ended third.

"Hopefully, we'll find a bit more tomorrow, otherwise we could find ourselves in trouble," added the Briton who leads Rosberg in the championship by 10 points going into his home race.

Hamilton won at Silverstone last year, on the way to taking his second title, and can count on the support of most of the expected 140,000 crowd on Sunday.

If they were disappointed not to see him dominant from the outset, they need only remember how he came back from the disappointment of misjudging the weather and qualifying sixth last year.