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World champion Lewis Hamilton says Ferrari pair Kimi Raikkonen and Sebastian Vettel pose the greatest threat to Mercedes in Sunday's Chinese Grand Prix.
Raikkonen and Vettel topped second practice at the Chinese Grand Prix, ahead of the Mercedes pair of Nico Rosberg and Hamilton.
Raikkonen was 0.109 seconds clear of Vettel, with Rosberg 0.237secs off the pace and Hamilton a further 0.196 seconds behind.
Red Bull's Daniel Ricciardo was fifth ahead of Toro Rosso's Max Verstappen in sixth.
"Ferrari are looking quite quick so we have a race," Hamilton, 31, said.
"I had a difficult day, we were struggling with the balance and it was a bit windy.
"We’ll have a bigger fight than we’ve ever had – which will be good for the fans."
Vettel says the Ferrari car can only get quicker, and hopes to challenge Rosberg in Sunday's race at the Shanghai International Circuit.
"It’s only Friday so I wouldn’t put too much importance on the times. The balance isn’t exactly how I wanted it,” the four-time world champion said.
"If we get everything right we have a chance."
Hamilton, who trails Mercedes team-mate Rosberg by 17 points in the championship table after the first two races of the season. faces a five-place grid penalty in China for changing the gearbox that was damaged in Bahrain.
F1 rules state that drivers must use a single gearbox for six consecutive races, but Mercedes decided to change the gearbox in Bahrain as taking the penalty in Shanghai gives the team the best chance of minimising the punishment.
Shangai has the longest straight of all tracks on the calendar, which gives Mercedes and Hamilton an opportunity to overtake other drivers and claw back the penalty.