Red Bull's Max Verstappen topped final practice at the US Grand Prix ahead of Ferrari's Sebastian Vettel and McLaren's Lando Norris.
Mercedes driver Lewis Hamilton is poised to secure a sixth world title on Sunday, but he could manage only fifth fastest, 0.618 seconds off the pace.
His team-mate Valtteri Bottas was fourth, but he was also some way off the front, 0.599secs behind Verstappen.
But Norris was the star of the session as he mixed it with the big teams.
The 20-year-old was a remarkable 0.513secs off the pace, with his team-mate Carlos Sainz nearly 0.6secs behind him in a more normal seventh place.
Charles Leclerc, the quicker of the two Ferrari drivers on Friday, missed the session when he suffered an engine failure on his first lap out of the pits.
Ferrari managed to get the car back to the garage with half an hour of the session remaining but face a race against time to prepare it in time for qualifying at 16:00 local time, 21:00 GMT.
The attention this weekend, though, is all on Hamilton, who will win the title as long as he does not lose more than 22 points to team-mate Bottas.
That means the Finn needs to win to have any chance of taking the championship to the next race in Brazil, but even then Hamilton can finish eighth and become champion.
Effectively, Hamilton will be champion unless Bottas wins and the Briton retires from the race.
But Mercedes appear to be on the back foot going into qualifying, a common occurrence in the second part of the season, in which Ferrari have established themselves as the fastest team over one lap.
Verstappen took pole position in Mexico a week ago, but was demoted to fourth on the grid for ignoring yellow warning flags at the end of qualifying.
It remains to be seen whether Red Bull can retain their advantage when Ferrari and Red Bull switch their engines to full power in qualifying.
Verstappen's team-mate Alexander Albon was sixth, 0.678secs off the Dutchman, with Alfa Romeo's Kimi Raikkonen eighth and the Toro Rosso of Pierre Gasly and Daniel Ricciardo's Renault completing the top 10.