Lewis Hamilton was surprised by Sebastian Vettel's "very quick" pace in Friday's practice for the US Grand Prix.
While many of his rivals struggled on the slippery surface at the new Circuit of the Americas, Vettel managed to put in late times in both sessions to blitz the field.
In FP1, he was 1.418s up on Hamilton while in the afternoon he pulled seven-tenths clear of Mark Webber. Hamilton was fourth, 1.030s adrift.
"I don't know where he got that time from, but that's very quick," the McLaren driver told Autosport.
"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."
But while the Brit may not have enjoyed the deficit to Vettel, he did enjoy his time out on the new track, which he hailed as a "great drivers' track - it's very tough to pull together three perfect sectors.
"Personally, I find the final two sectors easier than the first; those high-speed changes of direction need a good set-up, and balancing that requirement against the demands of the slow-speed stuff is tricky.
"Turn One probably looks more exciting from the outside than it does from inside the cockpit - it's certainly not easy, though. You can get oversteer, or lock up, and it's tricky to get a perfect line through there."