Champion-elect Lewis Hamilton finished fastest in second practice for the United States Grand Prix.
The British driver, who is set to win his sixth world title in Austin on Sunday, ended the day's final running leading the way from Ferrari's Charles Leclerc. Red Bull driver Max Verstappen finished third.
Hamilton will clinch this season's championship if he finishes at least eighth on Sunday. Valtteri Bottas, fifth fastest in practice here, must win to stand any chance of taking the title fight on to the penultimate round in Brazil.
Hamilton, boosted by the return of his long-standing race engineer Pete Bonnington following illness, has won on five of his previous seven visits to Austin.
And on the evidence of practice, the Mercedes driver appears in a strong position to see out the championship in style.
Mercedes underplayed their hand in the opening running on Friday – Hamilton finished only eighth – but the Briton ended the day three tenths clear of Leclerc.
Verstappen finished 0.315 sec down on Hamilton, with Ferrari's Sebastian Vettel – who slid backwards through the gravel trap at the right-handed turn 14 – fourth, 0.658 sec off the pace.
Bottas finished the best part of eight tenths down on his Mercedes team-mate. Red Bull driver Alex Albon was sixth.
Romain Grosjean's early crash brought a temporary halt to the afternoon running.
The French driver lost control of his Haas through the high-speed Esses with just 10 minutes on the clock.
Grosjean ended up in the barriers, damaging the front end of his car. He emerged unscathed from the accident, but the red flags were swiftly deployed to deal with his wounded machine. The action was suspended for almost 10 minutes.
British rookie Lando Norris ended the session in 14th, six places behind his McLaren team-mate Carlos Sainz.
George Russell made way for Canadian reserve driver Nicholas Latifi in the first session here, but returned later in the day to occupy his familiar 19th position, half-a-second faster than Robert Kubica in the sister Williams.