Lewis Hamilton fired an ominous warning to Sebastian Vettel after he delivered a practice double for the Japanese Grand Prix.

The British driver, who holds a dominant 50-point championship lead over Vettel with just five rounds of the season remaining, was fastest in the opening run before returning to the Suzuka circuit to top the time sheets in the afternoon session.

Hamilton was the best part of half-a-second clear of any other driver with Valtteri Bottas completing a Mercedes one-two, while Vettel was a distant third.

The Ferrari driver finished 0.833 seconds slower than his rival to suggest he will have his work cut out to prevent Hamilton from galloping to the chequered flag this weekend.

Hamilton has won five of the last six grands prix this season, and three of the last four to be staged in Japan, and the world champion was again in impressive form on Friday.

He posted a fastest lap of one minute and 28.217 seconds to finish 0.461sec clear of Bottas in the sister Mercedes.

"This track is awesome," a giddy Hamilton said over the radio. "I am having the best day."

"I am glad you're enjoying it," his engineer, Pete Bonnington, replied.

One man unlikely to be revelling in Hamilton's fine form is Vettel, who heads into the race with the title battle out of his hands for the first time this year.

He was a full second slower than Hamilton in the first session before fractionally reducing the deficit to the Mercedes driver in the second run.

But after boasting the superior machinery for much of the year, Ferrari are now behind Mercedes, and significantly slower than their rivals in straight-line power, too.

Indeed, Kimi Raikkonen, who finished fifth, was 1.28sec off Hamilton's pace with Red Bull driver Max Verstappen splitting the Ferrari duo.

Despite his blistering lap times, there was a hairy moment for Hamilton when he almost collided with the Toro Rosso of Pierre Gasly at the chicane in the first practice session.

Gasly was dawdling on the racing line, and Hamilton had to take evasive action at nearly 200mph, running off the circuit and gesticulating at the Frenchman.

Gasly was hauled in front of the stewards and given a reprimand for driving unnecessarily slowly.

There was drama for Fernando Alonso, too, after the Spaniard lost control of his McLaren on the entry of the high-speed Degner curve before he spun backwards and into the gravel.

Alonso avoided contact with the barriers and limped back to the garage with bits of sand falling out of his McLaren.

He ended the day in 17th, while Lando Norris, the British teenager who is being promoted to the race team next year, finished last after he deputised for Stoffel Vandoorne in the first session.

Vandoorne also finished bottom of the pile when he returned to the car in the afternoon.