Lewis Hamilton started his US Grand Prix weekend as he means to go on with an imposing fastest time in a wet first practice session.
The Mercedes driver, who can clinch a fifth title on Sunday, was 1.3 seconds clear of team-mate Valtteri Bottas.
Hamilton will be champion if he scores eight points more than Ferrari's Sebastian Vettel, who was fifth, nearly two seconds off the pace.
Vettel has been summoned to stewards over failing to slow for the red flag.
If found guilty, precedent suggests that could be a three-place grid penalty.
Red Bull's Max Verstappen and Daniel Ricciardo were third and fourth.
The day started with torrential rain in south Texas, but it had eased by the start of the first 90-minute session and cars were able to run from the start.
Red Bull set the early pace, before Bottas displaced them, only for Hamilton to move the goalposts on only his second flying lap.
The Finn was on a lap that would have been closer to Hamilton but pitted before the end of it and did not run again.
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The Ferraris did a lot more running than Mercedes - Hamilton did only six laps in total, and only two flying laps, while Vettel and Raikkonen did 17 and 19 laps each.
Ferrari have an aerodynamic upgrade at this race, as they attempt to claw back some of the performance they have lost to Mercedes in recent races.
The wet weather is expected to continue through the day, with a 60% chance of rain for the second session which starts at 14:00 local (20:00 UK time).
Behind Kimi Raikkonen's Ferrari in sixth place, Renault's Carlos Sainz headed what has come to be known a Formula 1's "B class", from the Haas of Romain Grosjean and Sauber's Charles Leclerc.
Leclerc caused a red flag part-way through the session when he spun at Turn Eight and spread gravel over the track on the exit of Turn Nine as he rejoined the track.
Outside the top 10, Britain's Lando Norris was just 0.196secs behind McLaren team-mate Fernando Alonso. The 18-year-old, who has been signed to race for McLaren in 2019, was standing in for regular driver Stoffel Vandoorne in the first session.