Sebastian Vettel led the first practice session at the Circuit of the Americas by over a second.
His final two laps were the quickest of the session, the last putting him 1.4s clear of second-placed Lewis Hamilton.
Hamilton led the way for much of the session but pitted a few laps before the end reporting a loss of grip from his front-left tyre. Fernando Alonso was third fastest for Ferrari, 2.2s slower than Vettel.
Lap times improved quickly throughout the 90 minutes of practice on the slippery track. Drivers reported very little grip when the session began.
Turn 19 was the most common scene of problems with driver after driver running wide at the corner. Vettel went off there more than once and Felipe Massa was one of several other drivers to have problems at the corner.
Nico Hulkenberg had two spins in his Force India, one of them at turn one at the top of the hill.
Ma Qing Hua was the only non-race driver to participate in the session, taking over from Narain Karthikeyan at HRT. He ended the session 1.7s slower than team mate Pedro de la Rosa.
2012 United States Grand Prix
Image © Red Bull/Getty images