Max Verstappen clocked the fastest time for Red Bull as champion-elect Lewis Hamilton of Mercedes finished up only fifth in Saturday morning's third and final free practice session at the United States Grand Prix.
The Dutchman, stripped of pole for ignoring yellow flags in Mexico last weekend, clocked a best lap in one minute and 33.305 seconds to outpace Ferrari's Sebastian Vettel by two-tenths on a dry day at the Circuit of the Americas.
Vettel's team-mate Charles Leclerc was unable to clock a time in the session after an early engine failure and he remained in the Ferrari garage as the team worked on his car.
Lando Norris was a surprise third for McLaren ahead of Valtteri Bottas, of Mercedes, and Hamilton, who seemed slightly out of sorts and almost snappy in some of his dialogue with race engineer Peter 'Bono' Bonnington.
Hamilton needs only to finish in the top eight, or for Bottas to fail to win Sunday's race, to clinch his sixth drivers' world title, but it seemed the pressure of seeking to succeed in style with another victory was affecting him.
Alex Albon, in the second Red Bull, was a strong sixth ahead of Carlos Sainz in the second McLaren, Kimi Raikkonen of Alfa Romeo, Pierre Gasly of Toro Rosso and Renault's Daniel Ricciardo.
On a welcome warmer morning in Texas, Romain Grosjean set the first meaningful lap time before Leclerc pulled up and parked.
"There's something wrong with the engine," he reported back to Ferrari. A brief Virtual Safety Car (VSC) period ensued.
Raikkonen was briefly on top before Vettel took over, trading fastest laps with Verstappen, before Bottas announced his intentions with a lap in 1:33.904, lifting him two-tenths beyond the Red Bull driver.
Hamilton produced fast sectors, but not a complete lap at his best and was unable to match Vettel when the German delivered three 'purple' sectors, denoting fastest times, to go top again in 1:33.523 with 12 minutes remaining.
Seemingly unperturbed, Verstappen retorted with 1:33.305 as Norris, unexpectedly, jumped to third for McLaren ahead of the Mercedes men.
Bottas was fourth and Hamilton fifth and they were briefly split by Albon before the champion bounced back, leaving most observers wondering if Mercedes were holding some speed back for qualifying later Saturday.