Charles Leclerc set the pace ahead of George Russell in an intriguing Miami GP opening practice as the first F1 session around America's new circuit saw Ferrari continue their 2022 form and Mercedes find theirs.
With drivers testing the limits of a 19-corner street track that surrounds the Miami Dolphins' Hard Rock Stadium, there were incidents aplenty in the opening hour - with Valtteri Bottas crashing his Alfa Romeo - and the lead changed several times before Leclerc made his advantage stick late on.
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Leclerc set the pace with a 1:31.098 in the Ferrari, with the championship-leading driver and team bouncing back after a difficult Emilia Romagna GP.
Red Bull's Max Verstappen had been fastest before dropping to third, with the title rivals pace no surprise.
But the notable pace from Practice One came from Russell and Mercedes, who enjoyed an encouraging start to their Miami GP weekend.
Russell, driving an upgraded Mercedes as the team desperately try to fix their porpoising and salvage their season, was just 0.071s off Leclerc. Lewis Hamilton was only eighth, although had a compromised final lap.
"I think Mercedes are in play this weekend," said Sky F1's Karun Chandhok. "You can start to see them getting their season back on track."
"For the first time this season, we could have three teams in play for pole position rather than two."
Mercedes are a distant third in the championship while Lewis Hamilton insists the title is already out of reach. But Martin Brundle added: "They are much faster and closer than they have been at any other point in the season.
"It is early doors we don't know how much fuel they are running but they are definitely much closer to being back in business."
Sergio Perez was fourth in the other Red Bull ahead of Pierre Gasly, with Carlos Sainz sixth in the other Ferrari.
Another surprise from first practice came from Williams and Alex Albon, who placed seventh and flirted with the top positions with his 1:31.8.
McLaren's Daniel Ricciardo and Lando Norris placed 11th.
An eagerly-anticipated F1 debut in Miami was always likely to see red flags and crashes, meanwhile, because of the narrow walls and demanding, tight circuit.
It was Bottas who caused the first red flag of the weekend when losing control of his Alfa Romeo through the tricky Turn 7, smashing his rear into the barriers. Bottas was unharmed, although his team have an early repair job on their hands.
The weekend continues with Practice Two later on Friday, with the session beginning at 10.30pm on Sky Sports F1.