The Monza pole sitter topped the session with a time of 1m32.120s amid the soft tire phase, which withstood a siege from Leclerc despite Sainz being outgunned in the first and second sectors.
As FP2 took place at the same time of day as qualifying and the race, it was considered the only representative practice session as the track was cooled under the night sky.
Leclerc set the pace on the medium-weight compound with his final run on the yellow-walled tires, erasing his teammate Sainz’s lead at the front by about a quarter of a second, as the Spaniard had largely been at the top of the timing boards in the early stages of the session.
Sainz had previously briefly eclipsed leader Fernando Alonso with a time of 1:33.303, which he reduced to 1:33.213 on his next flying run, but Leclerc was first under the 1:33 with 1:32.974 -Brand.
This was preceded by a short running break before the field returned to the track on the soft compound to simulate their qualifying runs as track conditions continued to cool.
Sainz was ahead of Leclerc on the road and had to be narrowly overtaken by the Monegasque in the first two sectors, but still set the record of 1:32.120 minutes.
Then Leclerc suffered an accident in turns 18 and 19 and swayed over the curb just before crossing the start-finish line, costing him the small lead he had earned and dropping him 0.018 seconds behind.
The Ferraris completed a second lap on the soft tires but neither driver improved and their times remained at the top of the timing boards for the remainder of the session.
George Russell came closest, being 0.235 seconds off the pace despite some erratic moments behind the wheel of his Mercedes W14. His performance pushed Alonso out of the top three as the two-time champion had to settle for fourth place in the overall standings.
Four-time Singapore Grand Prix winner Lewis Hamilton was fifth ahead of McLaren’s Lando Norris, who was fully equipped with the Woking outfit’s latest upgrade package.
In the meantime, the Red Bulls were unable to find any significant pace at the front and, above all, had to struggle with an unpredictable bottom. On one of his early runs on the medium tire, Verstappen failed to get his RB19 into the apex of Turn 13 and then ran wide on the exit as he lit up the rear wheels.
Teammate Sergio Perez also struggled with a nervous car, complaining that “I feel like I’m falling in every braking zone”, but still overtook his championship-leading teammate.
Nordic duo Kevin Magnussen and Valtteri Bottas completed the top 10, with the Finn acting as the meat in the Haas sandwich, while Nico Hulkenberg finished 11th overall.
Alex Albon confirmed the order by setting his fastest lap on the hard tyres, as a drive unit issue meant the London-born Thai was unable to complete a representative qualifying simulation on the soft tyres.