Lewis Hamilton admitted he was surprised by the pace of defending champion Sebastian Vettel in the opening free practice sessions ahead of tomorrow's United States Grand Prix.
The 25-year-old German topped both sessions for Red Bull, despite technical problems as he prepared for his 100th Formula One race, in which he hopes to win and become the youngest triple champion in the sport's history.
Vettel deposed Hamilton of McLaren with a late time 1.418 seconds quicker than the second-placed McLaren driver in practice one and then topped practice two by 0.757 seconds, despite losing an hour of running due to a water leak.
Hamilton said: "I don't know where he got that time from, but that's very quick.
"I don't know whether or not we have that pace, but we're going to go into tomorrow and make some changes - and we can definitely improve our car. We tested something there that wasn't so good. Hopefully we can go back to what we had in P1 and then it will be better tomorrow."
Hamilton's McLaren team-mate and fellow Briton Jenson Button was more relaxed about Vettel's speed, suggesting that other drivers had not shown their full potential.
He said: "I don't think many people actually got a good lap in in practice on low fuel, but the Red Bull's a very quick car and Seb's very quick in it.
"They're definitely going to be the people to beat in qualifying, but I don't think the gap is as big as it looks."
Button remained confident that Vettel's margin was illusory.
"I don't think there's going to be a one-second gap between Seb and us in qualifying, I think it's going to be closer than that," he said.
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