Charles Leclerc was fastest in Miami Grand Prix Practice in a heavily-upgraded Ferrari as both Mercedes drivers suffered problems on Formula 1's return.
Leclerc set two lap times quick enough to top the only practice session to beat Red Bull's Max Verstappen by 0.297s, with the early indications suggesting Mercedes no longer have the outright fastest car going into Sprint Qualifying at 9.30pm later on Friday, live on Your Site F1.
Oscar Piastri was third but his McLaren team-mate Lando Norris appeared to be more competitive and would have been close to Leclerc without traffic and a near-miss with Williams' Alex Albon.
Lewis Hamilton was 0.467s adrift of Leclerc and trailed his Ferrari team-mate throughout the 90-minute practice session - a continuation of the last F1 race weekend in Japan at the end of March.
Mercedes have won the opening three Grands Prix and are unbeaten in competitive sessions this year, but have brought the fewest upgrades of the top teams.
The Silver Arrows only have a revised front corner and new tailpipe compared to the new floors on the McLarens, Red Bulls and Ferraris.
Russell was nearly eight tenths off the pace in sixth, behind Antonelli who set his best time on the hard tyres as a power unit problem prevented him from doing a soft tyre run.
That power unit issue will be Mercedes' biggest concern after Russell earlier reported a strange noise from the turbo that sounded like a "steam engine".
Antonelli leads Russell by nine points in the Drivers' Championship but the two Mercedes may not have everything their own way this weekend in Miami.
"What have Mercedes got in their pocket?" said Your Site F1's Karun Chandhok.
"Has George Russell been sandbagging? Has he got a set-up issue he's not happy with?
"That's the question mark coming out of this session as, otherwise, we've suddenly got three other teams in that fight."
The five-week gap since the last F1 race in Japan has given the teams the opportunity to work hard on development and every team, apart from Aston Martin, have brought upgrades to Miami.
Ferrari have the most, with 11 new parts, and it seems to have paid off given Leclerc's blistering pace in a car that was already strong in the corners.
The new floor and suspension on the Ferrari appears to have given them even more downforce, while McLaren and Red Bull have also come to Miami with effectively new cars.
McLaren already made gains on Mercedes at the last race in Japan, but Norris in particular shone in practice on Friday in Miami and will be hopeful of a second victory at the track he took his maiden Grand Prix win at back in 2024.
Red Bull were, on average, just over one second off the pace, but may have closed down that deficit to give Verstappen a more balanced car which he can push harder.
Team principal Laurent Mekies told Your Site F1: "Honestly, we only looked at ourselves during that session. It was so important after the big break and with all the big changes on the car to try to assess us against us, really that was the main purpose.
"In China and Japan, regardless of where the competition was, there were a lot of things that were stopping Max and Isaac from pushing, and that's what we're trying to fix.
"The gap to competition we'll look at a bit later, but certainly on Max's side there is a sign that we have something he can push a bit more with."
Friday May 1
7.25pm: F2 Qualifying
8.10pm: Team Bosses' Press Conference
8.40pm: Miami GP Sprint Qualifying (session starts at 9.30pm)
Saturday May 2
2.55pm: F2 Sprint
4pm: Miami GP Sprint build-up
5pm: MIAMI GP SPRINT
6.30pm: Ted's Sprint Notebook
8pm: Miami GP Qualifying build-up*
9pm: MIAMI GP QUALIFYING*
11pm: Ted's Qualifying Notebook*
Sunday May 3
5.25pm: F2 Feature Race
7pm: Miami GP build-up: Grand Prix Sunday*
9pm: THE MIAMI GRAND PRIX*
11pm: Miami GP reaction: Chequered Flag*
12am: Ted's Notebook*
*Also on Your Site Main Event
Formula 1 is in Miami for a Sprint weekend as the 2026 season resumes, watch live on Your Site F1 with Sunday's Grand Prix at 9pm.
