Honda's Marc Marquez topped the opening day of MotoGP testing in Malaysia on his first time on the RC213V since undergoing surgery on a dislocated shoulder in December.
The reigning world champion kept his running to a minimum as he continues his recovery, and set the best lap of the day of 1:59.621s early in the afternoon.
Pol Espargaro used his track time from last weekend's shakedown test to good effect in the opening stages of Wednesday's running, the KTM rider hitting the top of the timesheets with a 2:00.313s.
This remained the benchmark as the first hour concluded, though shortly afterwards new works Ducati rider Danilo Petrucci moved just under three tenths clear as development work resumed on the GP19.
It would be over an hour before another change at the head of field was seen, with Alex Rins moving his Suzuki to the first sub-2:00 lap of the day with a 1:59.880s.
Wednesday's running marked just the second time since undergoing surgery on his left shoulder Marquez had been on a motorcycle, and as such his time on the Honda was limited.
Completing just 29 laps, Marquez still guided his RC213V to the top of the pile halfway through the session with a 1:59.621s.This remained unchallenged through to the chequered flag, with Rins trailing the Honda rider on the timesheets by 0.259 seconds.
Maverick Vinales was third fastest on the new look factory Yamaha. The Valencia test pacesetter once again has two engines to evaluate this test, updated from the original versions debuted last November.
Vinales was just over half a tenth outside of Rins' lap, with team-mate Valentino Rossi 0.117s further adrift in sixth spot.
Avintia's Tito Rabat, who made his MotoGP return from a horror leg break during testing last November, leaped up to fourth on his GP18 in the closing minutes of the session, edging Petrucci to end Wednesday as top Ducati rider.
Takaaki Nakagami, fastest in the Jerez test last year, was seventh quickest on his year-old LCR RC213V, with 2018 runner-up Andrea Dovizioso [Ducati], Jorge Lorenzo's stand-in Stefan Bradl [Honda] and Espargaro – who had a late crash – completing the top 10.
Franco Morbidelli [Petronas Sepang Racing] was 12th fastest on his first day with the 2019 Yamaha, the Italian edging ahead of top Aprilia runner Aleix Espargaro. Team-mate Andrea Iannone crashed on his out-lap at the start of the day.
Cal Crutchlow made his return to action for the first time since badly breaking his right ankle at Phillip Island, and was just over a second off the pace in 14th after 51 laps on an LCR Honda now fitted with a thumb-operated rear break to compensate for the restricted movement in his right foot.
Pramac's Francesco Bagnaia was top rookie in 15th, three tenths clear of Tech3's Miguel Oliveria, who suffered an early off. KTM stablemate Johann Zarco was a lowly 20th.
