Interlagos hosts Formula One's final sprint weekend of the season, with qualifying for Sunday's grand prix later on Friday while Saturday features the sprint shootout and a standalone 100km race.
Sainz lapped with a best time of one minute 11.732 seconds, with Leclerc - polesitter for the past two races - 0.108 slower and both after late runs on the fastest soft tyres on a windy afternoon.
George Russell, last year's winner in Brazil for Mercedes, was third fastest and 0.133 off the pace, but on the slower medium tyres, with Nico Hulkenberg fourth for Haas on softs.
Russell had been top of the timesheets until Ferrari bolted on the soft tyres near the end of the hour.
Triple world champion Max Verstappen was only 16th fastest, with Red Bull teammate Sergio Perez 18th but both set their times on the hard tyre.
Seven-times world champion Lewis Hamilton, only 20 points off Perez in the battle for second place in a season dominated by Verstappen, was 12th with a best time on the medium tyres and without doing a full lap on softs.
McLaren brought up the rear with Lando Norris 19th and Oscar Piastri 20th in what looked like a strategic approach with their times set on hard tyres and Piastri aborting a quick lap on the softs.
Norris and Hulkenberg banged wheels in an incident that was being investigated by stewards.
