Red Bull set the pace in the first day of F1's pre-season testing in Bahrain, but Mercedes suffered teething problems with their new car.
Conditions were tough, especially in the afternoon's session after a sandstorm blew across the Sakhir circuit and coated the track.
It did not put off Max Verstappen though, who set the pace with a 1:30.674 as he racked up 138 laps in the RB16B as he took sole control of the car on Friday. Team-mate Sergio Perez gets into the car for the first time on Saturday.
McLaren's Lando Norris got used to Mercedes power for the first time since 2014 with a 1:30.889 for the second fastest time of the day, while team-mate Daniel Ricciardo put the first mileage into his new car in the morning as the Woking-based squad pumped in almost 100 laps.
World champions Mercedes though suffered gearbox issues with the W12, as Valtteri Bottas could only complete six laps in the morning session, before Lewis Hamilton took over duties in the afternoon and was only able to put in 42 laps and was 2.2 seconds off Verstappen's pace, good enough for 10th.
Esteban Ocon put in more than 120 laps into the new Alpine A521 and set the third best time, with Lance Stroll fourth quickest in the newly rebranded Aston Martin, formerly Racing Point.
Charles Leclerc stopped out on track at the end of the first session in the Ferrari, while new team-mate Carlos Sainz Jnr got used to his surroundings in red, setting the fifth quickest time.
Antonio Giovinazzi and Kimi Raikkonen split duties for Alfa Romeo, and Yuki Tsunoda also put in his first testing miles after being promoted into Alpha Tauri alongside Pierre Gasly.
Test driver Roy Nissany put the first miles into the new Williams and set a quicker pace than both Haas newboys Nikita Mazapin and Mick Schumacher - son of seven-time world champion Michael - as they only out-paced teh hapless Bottas.