Lewis Hamilton admitted he was taken aback by Sebastian Vettel's pace as the Red Bull driver dominated both Friday practice sessions for the United States Grand Prix at Austin.
Vettel deposed Hamilton with a late time 1.418 seconds quicker than the second-placed McLaren driver in practice one, and then topped practice two by 0.757sec despite losing an hour of running to a water leak.
"I don't know where he got that time from, but that's very quick," said Hamilton. "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 teammate Jenson Button was more relaxed about Vettel's speed, suggesting that other drivers had not shown their full potential.
"I don't think many people actually got a good lap in in practice on low fuel," Button suggested. "But the Red Bull's a very quick car and Seb's very quick in it. They're going to definitely be the people to beat in qualifying, but I don't think the gap is as big as it looks."
Button's main concern was that Red Bull did not appear to be suffering from the severe tire warm-up issues that many teams were experiencing.
"It's not just their outright speed, it's how easily they can get the tires working," he said. "They can do a very quick lap on lap one, and for us that's impossible."
Still, he remained confident that Vettel's Friday 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," Button insisted.
The McLarens are running in their regular colors in Austin this weekend but with the signage for primary sponsor Vodaphone replaced by U.S.-based Verizon Wireless, of which Vodaphone owns a 45 percent stake of shares.