Andrea Dovizioso led Valentino Rossi by 0.070 seconds in opening practice for the MotoGP Malaysian Grand Prix, as Jorge Lorenzo made his first appearance since Motegi FP1.
Maverick Vinales led for a good portion of the session on his factory Yamaha, though was demoted in a late surge of soft tyre time attacks, with double Sepang winner Dovizioso emerging fastest of all with a 1:59.697s.
World champion Marc Marquez set the benchmark pace early on with a 2:01.160s, putting him six tenths out of reach of the field briefly before Vinales closed that gap down to just over a tenth and a half 10 minutes in.
The Honda rider moved the goalposts moments later with a 2:00.833s, with Vinales shadowing him by 0.012s, taking over from his fellow Spaniard with 24 minutes remaining with a 2:00.453s.
The four-tenth buffer Vinales built up with his effort midway through the session was only slightly dented by Marquez, though Pramac's Jack Miller mounted a late charge on fresh soft rubber and displaced the Yamaha man with a 1:59.969s.
Miller further improved to a 1:59.941s, but was usurped by Ducati stablemate Dovizioso with a 1:59.697s, putting him half a second shy of the outright lap record.
Rossi, on the sister works Yamaha, put Dovizioso's time under major threat on his final lap, missing out in second by just seven hundredths after utilising a tow from Pramac's Danilo Petrucci.
Dovizioso held onto top spot at the chequered flag, while Vinales moved back up to third behind Rossi with his final effort of 1:59.928s.
Miller was shuffled back to fourth in the end ahead of teammate Petrucci, with Suzuki's Alex Rins sixth fastest at the end of FP1.
Aleix Espargaro continued to evaluate numerous new parts on his 2019 prototype Aprilia on Friday morning at Sepang, and was seventh ahead of Alvaro Bautista, who has returned to his Nieto Ducati after standing in for Lorenzo at Phillip Island.
Johann Zarco and Marquez, who did not do a time attack at the end of the session, completed the top 10, while there was a late fall for Avintia's Xavier Simeon [22nd] at Turn 14.
Cal Crutchlow's injury replacement at LCR Stefan Bradl makes his fourth appearance of the season and his first with LCR since leaving the team in 2015. The HRC test rider was 15th fastest at the end of FP1, 1.7s off the pace.
Lorenzo completed 13 laps aboard his GP18 Ducati as he makes his return from surgery on the left wrist he fractured in Thailand, and completed the 24-rider field, 3.5s off the pace of teammate Dovizioso.
Clearly riding in extreme discomfort, it is unclear if Lorenzo will carry on past FP1. Ducati test rider Michele Pirro is on stand-by to take his place should the Spaniard pull out
