Montmelo, Spain - Valtteri Bottas impressed in his debut drive for world champion Mercedes on Monday as the new Formula One pre-season got under way.
Bottas outworked the rest of the field with 79 laps during the morning session at the Circuit de Barcelona-Catalunya. That total surpasses the 66 laps of the Spanish Grand Prix held at the same venue each season.
The 27-year-old Finn also owned the fastest time until Ferrari driver Sebastian Vettel bettered Bottas' best effort by 0.378 seconds.
Bottas, formerly of Williams, inherited the driver's seat at Mercedes from world champion Nico Rosberg, who retired in December five days after clinching the title.
Bottas' team-mate and three-time former champion Lewis Hamilton will take over driving for the Silver Arrows during Monday's afternoon session.
While Bottas and Vettel took turns churning out the laps under sunny and dry conditions, McLaren and Red Bull endured inauspicious starts to the new campaign.
McLaren, Red Bull struggle
Fernando Alonso's McLaren was stuck in the garage following a sole trip around the track as its technicians reportedly tried to repair an oil-related problem with its Honda motor.
Red Bull driver Daniel Ricciardo didn't do much better, getting through just four laps before his new RB13 car ground to a halt near Turn 4 because of what the team called "a sensor issue." The stranded Australian was ferried back to the boxes under a red flag.
Neither Ricciardo nor Alonso got back behind the wheel before the end of the four-hour opening session.
The opening test will continue later Monday and run through Thursday.
Teams are trying out their cars after a rulebook overhaul produced new aerodynamic and tire regulations. The designs have led to wider vehicles, which increases downforce and grip with the goal of boosting speeds.
Mercedes' rivals are hoping the changes will help them make up ground on a team that has won 51 of the last 59 races en route to three consecutive driver and constructor titles.
To compare with times on the same track from last year, Vettel's fastest effort of 1 minute, 22.791 seconds on Monday marks an improvement on his own leading time of 1:22.810 he set over the four days of last year's opening test.
The track near Barcelona will host a second round of testing from March 7-10 before the season starts at the Australian Grand Prix on March 26.