Mercedes has revealed an eyebrow-raising car design just a week out from the start of the Formula 1 season.

F1 teams hit the track in Bahrain on Thursday evening AEDT for the first day of the final pre-season testing weekend before the opening grand prix on March 20.

But before an engine was fired up, Mercedes had heads turning in the paddock with its new car design.

F1 has undergone the largest regulation upheaval in four decades, with teams grappling with new designs for aerodynamics and tyres.

The new regulations have led to stark differences in the look of the cars as the teams try to find the best formula for speed and consistency.

Mercedes has taken its design to a new level, removing the sidepods from its vehicle.

Sidepods are designed to direct air inside the car, keeping the engine cool.

F1's motorsport managing director Ross Brawn said Mercedes' decision to remove them was an unexpected and "very extreme" interpretation of the new rules.

"Inevitably, there's going to be a lot of debate about their interpretation. That's what happens with new regulations," Brawn told F1 TV.

"However hard you try to close off all the options — and believe me we closed off hundreds of them — the innovation in Formula 1 is always extreme."

Tough day for McLaren as Riccardo misses out due to illness

McLaren had a frustrating day in Bahrain, with one driver out and the other spending plenty of time in the garage.

Australian Daniel Ricciardo was not at the track on Thursday due to illness.

Aside from missing out on a full day of testing, Ricciardo was also absent for the annual photograph to mark the start of the season.

Things were not much better for his teammate Lando Norris, who spent the first half of the day in the garage.

A brake cooling issue meant Norris was only able to drive 50 laps around the 5.4-kilometre track in Sakhir.

"[It was] a tough day with a few problems, which set us back a lot of laps, meaning we couldn't do the long runs," Norris said.

"Hopefully the next few days, we can make up for that. We just have to change the program around a little bit in how we're trying to understand the car."

Loading

Norris was sixth fastest on Thursday with a time of 1 minute 35.356 seconds.

Alpha Tauri driver Pierre Gasly topped the time sheets with 1:33.902, followed by the Ferrari pairing of Carlos Sainz and Charles Leclerc.