Title rivals Max Verstappen and Lando Norris ended the only practice session at the United States Grand Prix in third and fourth places.
Carlos Sainz set the pace ahead of Ferrari team-mate Charles Leclerc at the Circuit of the Americas in Austin.
Sainz was 0.021secs ahead of Leclerc and 0.253secs quicker than Verstappen, whose time was set earlier in the session than those of Ferrari and McLaren when the track would have been slower.
Norris, who starts the weekend 52 points behind Verstappen in the championship, was 0.013secs slower than the Dutchman.
Mercedes driver Lewis Hamilton, who had a high-speed spin at Turn Four earlier in the session, was fifth, ahead of team-mate George Russell, who himself spun at Turn One.
Mercedes are among the teams with a significant upgrade on their car, along with Red Bull, McLaren and Aston Martin.
The weekend has started with a controversy over a device on the Red Bull that can be used to adjust the height of the front of the floor.
Red Bull have responded by pointing out that McLaren have made changes to their rear wing – an extension of the conversations they had with governing body the FIA after the Azerbaijan Grand Prix, where it was seen to flex on the straight.
A McLaren spokesperson said: “As previously mentioned, McLaren proactively offered to make some minor adjustments to our rear wing following the Azerbaijan GP. We have made minor adjustments to all our rear wings since Baku to varying extents to ensure no further issues in this area.”
Liam Lawson, who has replaced Daniel Ricciardo at RB, was 13th, 0.130secs behind team-mate Yuki Tsunoda in 10th.
This is a ‘sprint’ weekend, with a shorter race on Saturday. Qualifying for this is at 22:30 BST on Friday, with the sprint on Saturday at 19:00.