Repsol Honda's Dani Pedrosa ended the opening day of MotoGP testing at the Sepang International Circuit fastest of all after a late effort of 1:59.427.
All 23 confirmed race riders, as well as several testers, have descended on the Sepang Circuit in Malaysia for the first of three official pre-season tests, ahead of the start of the 2018 campaign in Qatar in March.
Heavy rain in the morning left the track saturated, and the lengthy time the circuit took to dry meant only a handful of riders ventured out in the opening hour of the day.
Eventually a dry line began to appear, with LCR Honda's Cal Crutchlow – who has three 2018-spec RC213Vs at his disposal this week – the first slick tyre runner to go top of the pile, prompting many of his rivals to commence their respective programmes.
Testing a new engine, as well as various chassis and electronics changes, Valentino Rossi – who along with Yamaha team-mate Maverick Vinales and Tech3 rider Johann Zarco tested at Sepang in November – shot to the top of the standings halfway through the session with a 2m00.799s.
Around 35 minutes later, Vinales deposed the Italian, before 2017 championship runner-up Andrea Dovizioso took his Ducati GP18 to the top of the pile with a 2:00.556.
The session was briefly halted with just over 90 minutes on the clock after Angel Nieto Team rider Alvaro Bautista suffered a small fall, while Suzuki's Andrea Iannone ground to a halt at the side of the track just before the red flag was flown.
A flurry of fast laps with 75 minutes on the clock saw Pedrosa and Rossi swap fastest lap, the Italian's 2:00.233 standing as the best lap until the final quarter of an hour, when the Honda rider dropped into the 1:59s bracket.
Dovizioso ended the day 0.343s adrift in second, with his factory Ducati team-mate Jorge Lorenzo and the Pramac duo of Danilo Petrucci and Jack Miller completing the top five.
Rossi found himself shuffled back to sixth after calling it a day in the closing stages of the session, with reigning world champion Marc Marquez – who, like Crutchlow, had three RC213Vs to try – 0.057s behind the Italian in seventh.
Zarco headed KTM's Pol Espargaro to complete the top 10, while Suzuki's Alex Rins was 11th, ahead of LCR's Takaaki Nakagami, who took top rookie honours in 12th, followed by Vinales, Aprilia's Aleix Espargaro and Avintia's Tito Rabat.
Yonny Hernandez, who has been brought in for this test to replace the absent Jonas Folger, completed his first day on Yamaha machinery down in 23rd, with the 30-strong field rounded off by KTM test rider Mika Kallio.
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