Max Verstappen suggested he could join the fight for pole position in the US GP by setting the pace in Practice Three - as title-chasing Lewis Hamilton finished just fifth.
A week after claiming a pole position that ultimately wasn't in Mexico after a penalty, an impressive Verstappen outpaced nearest challenger Sebastian Vettel of Ferrari by 0.218s with a fastest lap of 1:33.305.
But Charles Leclerc, the polesitter at five of the last six races, ran into problems when he pulled up on track with his Ferrari trailing smoke from its airbox.
Ferrari traced the issue to an oil leak and will revert to a used engine on Leclerc's car for the remainder of the weekend, although he escapes a grid penalty for the change.
Qualifying begins at 7.10pm on Sky Sports F1, with build-up from 5.30pm.
Question marks surround Mercedes' prospects heading into qualifying after Valtteri Bottas and Hamilton finished 0.6s off the pace. Toto Wolff, however, seemed calm when speaking to Sky F1 after the session, suggesting there was more performance and engine power to come from the world champions in qualifying.
They were outpaced by McLaren's Lando Norris, who was just half a second back on his full Austin debut as the Woking squad finished with both cars in the top 10.
Kimi Raikkonen vaulted in to the top 10 for Alfa Romeo, who have slipped down the order in the second half of the season, while Pierre Gasly continued to run well for Toro Rosso in ninth.
Renault do not yet look to have the pace to challenge McLaren at the head of the midfield, but Daniel Ricciardo and Nico Hulkenberg do look capable of challenging for Q3 positions.
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