George Russell has finished fastest in second practice for the Miami Grand Prix as defending F1 champion Max Verstappen broke down in his Red Bull.

In sweltering heat, Russell was 0.106 seconds clear of championship leader Charles Leclerc, with Sergio Perez third and Lewis Hamilton fourth.

Carlos Sainz's poor run of form took another sorry twist when he crashed out in his Ferrari.

Formula One is in Miami for the first time, at a 5.4-km circuit constructed around the Hard Rock Stadium, 24 kms north of the city.

Mercedes have brought a number of upgrades to America, including a new low-drag rear wing and revised front wing, in a bid to reverse their early-season problems.

And the early signs are positive, with Russell improving on second in the opening running to post the day's fastest time.

Hamilton, 58 points behind Leclerc in the standings, was 0.241 sec adrift of his teammate.

"We don't really understand it," said Russell. "We have hit the ground running.

"We knew the conditions would suit us better here because this is the first real hot race of the season, and that has helped us, but it is only Friday and we are not getting carried away.

"But it has definitely been a productive day for us, and probably the most productive Friday we have had this year."

Hamilton added: "Practice is just practice and everyone is doing something different - not everyone is showing their true pace.

"But we seem to be quicker here. We have improved in some way, I am not sure where that is because we still have the bouncing.

"There are definitely positive elements to take from today. I am still struggling with the car but George looked fast out there."

Verstappen dominated in Imola a fortnight ago to get his championship defence back on track.

But Red Bull's mechanical woes, which saw the Dutchman retire from two of the opening three races, reared its head again on Friday.

The world champion missed the opening chunk of the one-hour running following a gearbox change, and when he eventually emerged on track, his rear brakes overheated and he was forced to limp back to the pits.

"It's shit," said Verstappen on the radio. "I couldn't f****** steer. I cannot do anything. I'm really sorry."

Team principal Christian Horner was seen shaking his head on the Red Bull pit wall.

Australian Daniel Ricciardo, who finished the first practice session in 10th, managed just 12th in the second practice session.

His teammate Lando Norris improved to sixth from 11th, in a somewhat promising performance for the underperforming team.

Over at Ferrari, there was further disappointment for Sainz.

The Spaniard crashed out on the opening laps of the past two rounds in Melbourne and Imola, and then suffered a front-right puncture in the first action after losing control of his Ferrari through Turns 4 and 5 and limping back to the pits.

But worse was to come for Sainz later in the day when he carried too much speed through Turn 13 and hit the wall.

Sainz sustained significant damage to the front-left of his machine with the action suspended to retrieve his stricken car.