Mercedes driver and reigning world champion Lewis Hamilton has topped the leaderboard in the first free practice session ahead of Sunday's season-opening grand prix, the first of two in Austria on successive weekends.

Hamilton, bidding to equal Michael Schumacher's record of seven world championships, clocked 1 minute 4.816 seconds around 4.318km circuit, finishing 0.356 seconds ahead of teammate Valtteri Bottas.

Hamilton's Mercedes team have dropped their traditional silver colours for black this season to show solidarity with their star driver in the fight against racism.

The champion team may not have everything their own way on the track, though, with Red Bull seeking clarification from F1's governing body about a controversial new dual axis steering (DAS) system being debuted by Mercedes.

"It's a complicated system, a clever system and obviously we're after some clarifications from the FIA," Red Bull team boss Christian Horner told Sky F1 television.

"It depends what it actually does and achieves. Everything has to earn its place on the car ... these rules are so complex, it's just understanding which parts of the regulations it fits."

Sunday's race will signal the latest ever start to an F1 championship after the COVID-19 pandemic forced the cancellation of the scheduled opener in Melbourne on March 15.

Friday was the first time all the drivers had been on track together since the end of pre-season testing in Barcelona in February.

The last time any of them raced was in Abu Dhabi in December last year.

"It's good to hear the engines and see cars again and almost get back to some phase of normality," Horner said.

The grand prix is being run under strict health and safety conditions, with teams operating in 'bubbles' and isolated from each other in a paddock, with drivers wearing face masks even when talking to the media by video link.

The race will be run completely without spectators for the first time in F1 history.

Australian Associated Press