The Briton missed almost 20 minutes of the session in Austin because of a hydraulics issue, although he still finished the day with the fastest time, beating Mercedes teammate Nico Rosberg by three-thousandths of a second.
The world championship leader told Sky Sports News: "There was fluid on the floor, a leak of some kind, but I don't think it is anything big. It's not helpful but Nico had a long run so as a team we will get data from that, and for me I'll get a better feel during the race.
"Nico is looking very quick, it is going to be very tough, but I am up for the battle. There will be many battles this weekend but he is the closest on time."
The team clinched the constructors' title at the Russian Grand Prix earlier this month.