Jack Miller, riding at only the second track this season (after Portimao) where he has some prior KTM experience, climbs to fifth.
Monster Yamaha team-mate Fabio Quartararo also fits new rubber (soft front, hard rear) to slot into second place.
But others are also out on new rubber, with Marini back into second and Augusto Fernandez third to demote Quartararo to 4th.
Morbidelli's charge, on fresh tyres, continues as he pulls 0.475s clear of Alex Marquez at the top.
Run, @AleixEspargaro!
— MotoGP™ (@MotoGP) November 10, 2023
Fire bursting out of his Aprilia bike after a crash #MalaysianGPpic.twitter.com/N1nmFW0FBd
Franco Morbidelli and Luca Marini are now up to second and third places, between Alex Marquez and Aleix Espargaro.
Espargaro initially ran over to the bike, but soon sprinted away when big flames began firing out of the exhaust.
More heat problems for Aprilia: Aleix Espargaro's RS-GP catches fire after falling at the final turn.
Halfway: Alex Marquez (1st), A. Espargaro, Martin, Bastianini, Morbidelli, Quartararo, P.Espargaro, M.Marquez, Miller, A.Fernandez, Zarco, B.Binder, Bezzecchi, Marini, Vinales, Oliveira, R.Fernandez, di Giannantonio, Mir, Nakagami, Bagnaia, Bautista, Lecuona.
Jorge Martin is running the front fork wings on his Pramac Ducati this morning, he remains in third, 0.3s behind Alex Marquez on the GP22.
With his factory Ducati future the subject of recent speculation, Enea Bastianini moves up to 4th. Team-mate Francesco Bagnaia is currently just 20th, continuing his pattern of a slow start to a race weekend.
After 15mins Alvaro Bautista is 22nd quickest, 2.291s behind Alex Marquez and ahead of fellow WorldSBK rider Iker Lecuona (LCR Honda).
Alex Marquez is now quickest for Gresini Ducati ahead of Aprilia's Aleix Espargaro and Yamaha's Fabio Quartararo.
Bautista has two crew chiefs this weekend: Usual Ducati MotoGP test team crew chief Marco Palmerini will be assisted by Bautista's WorldSBK crew chief Giulio Nava.
The Marquez brothers have set exactly the same lap time!
3 laps done and Martin is fastest with a 2m 0.984s ahead of Alex Marquez and Marc Marquez.
Alvaro Bautista is riding in his usual Aruba.it livery, on the factory GP23 that Ducati test rider Michele Pirro has been using in his wild-card appearances.
Early scare for Marco Bezzecchi who runs through the gravel at the final turn at considerable speed, but the rider with a recently broken collarbone fortunately stays upright.
Luca Marini set the fastest lap in February's pre-season test, with a 1m 57.889s.
Official Sepang MotoGP records
Best lap: Jorge Martin 1m 57.790s (2022)
Fastest race lap: Jorge Martin 1m 59.634s (2022)
The 45-minute opening practice session begins, all 23 riders leave the pits on soft front and hard rear tyres.
The hot conditions will be a worry for Aprilia, whose riders were roasted by the heat from the RS-GPs in the recent Thai MotoGP. The heat is not such an issue in practice, where riders are not in the pack, but some modifications are planned to try and reduce the heat problem in the races.
The current situation regarding tyre pressure warnings and penalties with three rounds to go...#MotoGP#MalaysianGPpic.twitter.com/YF89lup2VF
— Peter McLaren (@McLarenMotoGP) November 10, 2023
Typically hot and humid Sepang conditions will greet the MotoGP field for their opening practice session this morning, where the field will be joined by wild-card and reigning double WorldSBK champion Alvaro Bautista.