Yamaha's Maverick Vinales led the opening day of the Qatar pre-season test by 0.108 seconds, as Honda's Jorge Lorenzo returned to action a month after breaking his wrist.
Vinales, who topped day one of the Qatar test last year, took over at the top of the timesheets with a 1:55.051s with just over two hours remaining and remained unchallenged through to the chequered flag.
LCR's Cal Crutchlow led the way at the end of the opening hour of running on his 2019-spec Honda with a 2:00.399s, and was shadowed by Sepang Racing's Franco Morbidelli.
The satellite Yamaha rider soon deposed Crutchlow with a 1:58.011s, before reigning world champion Marc Marquez moved his works RC213V to the head of the field with a brace of session-best laps – the best of which a 1:57.487s.
Marquez would crashed at the final just over an hour later, but both rider and bike were unscathed, and he would return to action soon after to go onto finish 10th.
With around an hour and a half of the session gone, Sepang pacesetter Danilo Petrucci usurped Marquez on his factory Ducati with a 1:57.204s, which he duly improved to a 1:56.679s as he carried out chassis work on Saturday.
Petrucci would be dethroned by team-mate Andrea Dovizioso with just under four hours left on the clock, with last year's Qatar race winner edging 0.034s clear.
The Ducatis would trade top spot once more as the evening wore on, with Suzuki's Alex Rins – testing a new twin exhaust – and Vinales also involved in this battle for the fastest lap.
Vinales led the times heading into the final hour with a 1:55.051s and came under no threat, with Rins just over a tenth shy in second and Dovizioso top Ducati in third.
Petrucci completed the top four ahead of the sister works Yamaha of Valentino Rossi. Avintia's Tito Rabat headed SRT's Fabio Quartararo, with Takaaki Nakagami [LCR], KTM's Pol Espargaro and Marquez rounding out the top 10.
Joan Mir was just 0.097s outside of the top 10 on the second Suzuki, with Aprilia duo Aleix Espargaro and Andrea Iannone – who both crashed during the session – splitting Mir from Morbidelli and Pramac's Francesco Bagnaia.
Johan Zarco ended up a low-key 16th and 1.7s off the pace on his KTM, with stablemate Miguel Oliveira just over a tenth further back in 17th on the Tech3 RC16. Early pacesetter Crutchlow faded to 18th.
Jorge Lorenzo made his first appearance in Repsol colours and his first on the Honda since testing last November, and was just over two seconds off the pace in 21st at the end of his first session since breaking his wrist last month.
During Saturday's session, the riders trialled a new in-race penalty system for Race Direction, whereby riders ran through a route on the run-off area of Turn 6. This is intended to replace the current penalty of dropping one position for track limits infringements.
