Ferrari were fastest in Singapore Grand Prix practice on Friday as Red Bull's record 15-race unbeaten run looked in danger of ending.

Charles Leclerc was top of the timesheets in the first session with teammate Carlos Sainz second, with the positions reversed later as the Italian team again led one-two.

Red Bull's runaway F1 championship leader Max Verstappen, chasing an unprecedented 11th win in a row, was third and eighth respectively in the hot and humid sessions.

Teammate Sergio Perez, last year's winner on the Marina Bay layout, was seventh in both and struggling with the rear end of the car.

"Every braking zone I feel like I'm going to crash. The rear is just stepping out massively," the Mexican relayed over the team radio.

Big lizards are a familiar sight in equatorial Singapore but they startled drivers after straying onto the F1 racetrack during practice on Friday.

Yellow flags were waved to warn of the danger of hitting one of the creatures, which can grow as long as three meters, as they languidly crossed the tarmac at dusk during the hour-long session.

"It's going to be a brave marshal to go out and pick one of those up," Red Bull team boss Christian Horner told Sky Sports television. "Hopefully they'll be going to bed soon."

Local fauna are not uncommon company, with drivers used to groundhogs and foxes on Canada's Gilles Villeneuve circuit in Montreal. Malaysia, now no longer on the calendar, had an occasional problem with snakes.

Friday's lizard incursion recalled the 2016 Singapore race when Verstappen was surprised on track by a big one that his engineer dubbed "Godzilla."

"Ah. There's a lizard again on the track. A smaller one this time," he said over the team radio in Friday's session.

Singapore's National Parks' Board advises on its website that monitor lizards are typically shy, unless cornered, and to observe them from afar.

They are also diurnal, meaning they sleep at night, and will hopefully pose no further difficulty for drivers preparing for Sunday's night race.

"I think the one that Max met the last time was more of a Komodo dragon than a lizard, but the main thing is hopefully they can grab them up and put them somewhere safe and they don't get involved in the next session," said Horner.

(With input from agencies)