Ferrari's Charles Leclerc set the pace from Red Bull's Max Verstappen in first practice at the United States Grand Prix.
Leclerc was 0.084 seconds quicker than Verstappen at the start of the first weekend on which Mercedes driver Kimi Antonelli can clinch his first world title.
The Italian was fifth fastest, 0.481secs off the pace, behind McLaren's Lando Norris and Mercedes' George Russell.
Ferrari's Lewis Hamilton was sixth ahead of McLaren's Oscar Piastri.
Antonelli will win the championship in Austin only if he wins the race and Russell finishes below eighth and Hamilton below fourth.
Mercedes have reverted both their cars to the upgraded specification that was introduced in Malaysia two races ago but about which there remain questions over its relative performance.
The team believe that the upgrade has more performance within it, but that they have not yet found the appropriate set-up to maximise it. They hope a full, 'normal' weekend with three practice sessions will enable them to hone it.
Ferrari have an upgrade here which is fundamentally a new floor. Both Leclerc and Hamilton played down its potential impact.
Hamilton said: "We have a very, very small upgrade this weekend. It's less than a tenth. It's like half a tenth.
"So, I don't think that propels us forward enough to necessarily challenge that deficit. But maybe the track suits our car, and maybe they'll have a one-off weekend. Maybe Mercedes will have an off weekend again, and we'll be right there to take it."
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