The Ducati-Pramac rider beat his compatriot Aleix Espargaro of Aprilia into second with the Italian championship leader Francesco Bagnaia of Ducati in third.
After costly crashes in recent outings Martin has slipped to second in the overall MotoGP standings and needs 10 points more than Bagnaia, on 222pts, to retake the lead at this halfway stage of the season.
Ducati's Enea Bastianini was 4th ahead of the Australian Jack Miller on a KTM while the six-time MotoGP world champion Marc Marquez took tenth to book his ticket for the second qualifying session.
The ten fastest times in this session go directly into the second session on Saturday morning to decide the top twelve places on the starting grid for the sprint on Saturday and the GP itself on Sunday.
Practice
1. Jorge Martín (ESP/Ducati-Pramac) 1min 57.911sec, 2. Aleix Espargaro (ESP/Aprilia) at 0.045sec, 3. Francesco Bagnaia (ITA/Ducati) 0.119, 4. Enea Bastianini (ITA/Ducati) 0.279, 5. Jack Miller (AUS/KTM) 0.349, 6. Fabio Di Giannantonio (ITA/Ducati-VR46) 0.407, 7. Brad Binder (RSA/KTM) 0.475, 8. Maverick Vinales (ESP/Aprilia) 0.478, 9. Marco Bezzecchi (ITA/Ducati-VR46) 0.595, 10. Marc Marquez (ESP/Ducati-Gresini) 0.674
