Lewis Hamilton's preparations for an historic eighth world championship have been dealt a blow after his Mercedes team ran into significant trouble on the first day of Formula One testing in Bahrain.

With only three days of pre-season action – down from as many as eight in recent years – the grid is in a race against time to fine tune their machines for the new campaign.

But Hamilton's Mercedes completed the fewest laps in Sakhir on Friday after they were sidelined for the majority of the four-hour morning session with a gearbox failure.

Valtteri Bottas eventually emerged from the Mercedes garage with 35 minutes remaining before lunch and managed just six laps.

Hamilton took over the Mercedes driving duties in the afternoon, notching up an additional 42 laps for the world champions, but his running was disrupted by a sandstorm in the desert which affected visibility. The Briton, battling for grip, also ran off the circuit on a number of occasions.

The reigning champion finished 10th, 2.238 seconds adrift of Red Bull's Max Verstappen, who set the fastest time of the day, while completing more miles  – a staggering 139 laps – than any other driver.

Hamilton, who won 11 of last season's 17 races as he equalled Michael Schumacher's record of seven titles, will be back in the Mercedes cockpit on Saturday.

But team principal Toto Wolff has admitted the Silver Arrows can ill-afford any further setbacks ahead of the new campaign which gets under way in the Gulf Kingdom on March 28.

"It wasn't a good start because we had a gearbox issue that came out of nowhere which we couldn't identify and understand," said Wolff.

"If we're able to have a smoother ride from here onwards then we can recover.

"But if we have more stumbling blocks, then obviously with three days of testing there is not a lot we can do."

Mercedes, who are bidding for a remarkable eighth drivers' and constructors' double, have been bulletproof at testing in recent seasons.

So their troubles on Friday will come as a welcome boost to the chasing pack who are desperate to bridge the gap.

Ahead of the test, Mercedes were the only team to elect against a filming day, where drivers are permitted to complete 60 miles in their new car. The running would have enabled Mercedes to identify their gearbox failure.

Asked in hindsight if it was the wrong decision, Bottas said: "Now, yes, we would have done the filming day before. I am sure that will be reviewed for next year."

With few tweaks to the rulebook over the winter, after a major overhaul of the regulations was delayed until 2022 following the coronavirus pandemic, Hamilton's closest challenge is set to come in the form of Verstappen.

The Dutchman, 23, headed the time charts from Britain's Lando Norris by two tenths.

Four-time world champion Sebastian Vettel broke down at the end of the pit lane in his Aston Martin, while Mick Schumacher also ran into trouble.

Haas driver Schumacher, the son of seven-time world champion Michael, was restricted to just 15 laps following a gearbox change.