The new 2022 cars, and a modified circuit, meant that Sainz was more than five seconds under Michael Schumacher's 2004 lap record.
The Red Bull Racing duo took the next two places with Sergio Perez taking the third-fastest time - 0.022s behind Leclerc. He was just ahead of reigning world champion, Max Verstappen, who spent the opening part of the session doing aero testing on his Red Bull.
McLaren’s Lando Norris was an encouraging fifth fastest ahead of the first of the two Alpine cars, driven by Esteban Ocon and Fernando Alonso. Ocon had the better of his Spanish teammate Alonso, who spend part of the session off the track after briefly going through the gravel
Sebastian Vettel had a tough session as the four-time world champion, returned for the first time this season after contracting COVID-19. The German had to stop on the circuit as his Aston Martin caught fire. Vettel was kind enough to help the fire marshals douse the flames with a fire extinguisher.
Vettel capped off his recovery effort by sensationally returning back to the pitlane on a marshal's moped, waving to the crowd in the process. The Aston Martin team will have to change the power unit in time for the second one-hour practice session scheduled at 4:00pm.
Vettel, a three-time winner of the Australian Grand Prix, was classified 13th for the session, three places ahead of teammate Lance Stroll.
The F1 cars return to the track for Practice 2 later today.