George Russell put his revived Mercedes team back on top of the Formula One timesheets with the fastest lap in second practice for the inaugural Miami Grand Prix on Friday.
In a boost for the champions who have wrestled with a 'porpoising', or bouncing, car in the first four races, the Briton was 0.106 of a second faster than Ferrari's overall leader Charles Leclerc.
Russell's impressive time of one minute 29.938 seconds made Mercedes look more competitive than they have so far this campaign.
"We always knew that the warm conditions here in Miami would suit our car better – we’ve suffered with getting temperature in the tyres at previous races so that’s a big factor here," said Russell.
"The car is running well but it’s only Friday, we’re not getting carried away. It’s probably been the most productive Friday we’ve had this season in terms of learning," he added.
Leclerc had been 0.071 quicker than the Briton in the opening session with a lap of 1:31.098.
Red Bull's world champion Max Verstappen, who was third in first practice, failed to set a time in the second session after his car suffered a hydraulics problem and the brakes overheated.
"He's had a horrible day," said team boss Christian Horner. "It's frustrating not to have that track time."
Verstappen's teammate Sergio Perez was third fastest in practice two but was unimpressed with the Miami track.
"I am really disappointed there is no grip off-line. It’s a shame because I think the racing will be bad due to that. As soon as you try to go off-line, there is no grip," the Mexican said.
Leclerc and Verstappen have each won twice this season, with the Ferrari man 27 points clear of the Dutch 24-year-old whose efforts have been undermined by mechanical problems and retirements.
Mercedes have brought new parts in the hope of unlocking performance and first impressions were of clear improvement, with the car in a low-drag setup and looking faster without bouncing.
Russell's seven-time world champion team mate Lewis Hamilton was eighth and fourth fastest respectively in the two sessions.
"We still have the bouncing so we haven’t cured it but bit by bit, we’re improving the car," said Hamilton. "The race is going to be tough, particularly with the tyres overheating and it’s very hot for the drivers, I’ve already lost a couple of kilos today," he said.
The humid first session around the Miami Dolphins' Hard Rock Stadium was halted with 24 minutes to go when Alfa Romeo's Valtteri Bottas spun backwards into the tyre barrier at turn seven, damaging his car's rear wing. Action resumed after a 10-minute break but Bottas did not go out in the second practice.
Haas driver Mick Schumacher and McLaren's Daniel Ricciardo were summoned to the stewards after an earlier incident that saw the German narrowly avoid smashing into the rear of the Australian's car. The stewards decided to take no further action.
Alpine's Esteban Ocon was reprimanded for a pit lane near-miss with Russell after the Frenchman's team made an unsafe release.
Leclerc, bouncing back from a disappointing weekend at Ferrari's home Imola circuit two weeks ago, spun early on while team mate Carlos Sainz, sixth fastest, suffered a puncture after the red flag period. The Spaniard then crashed in the second session.
Williams' Alex Albon, seventh fastest, was fined €800 ($844) for speeding in the pit lane.
The second session was also briefly halted when Williams Nicholas Latifi stopped on track.
