The Monza polesitter punched in a 1m32.120s to top the session amid the soft tyre phase of running, which withstood a siege from Leclerc despite Sainz's inferior first and second sectors.
Being set at the same time of day as qualifying and the race, FP2 was considered the sole representative practice session as the track had cooled under the night's sky.
Leclerc set the pace on the medium compound with his final run on the yellow-walled tyres, overturning team-mate Sainz's advantage at the top by approximately a quarter of a second as the Spaniard had largely bossed the top of the timing boards in the early phases of the session.
Sainz had previously eclipsed brief headliner Fernando Alonso with a 1m33.303s, which he'd brought down to a 1m33.213s on his next flying run, but Leclerc was the first below the 1m33s bracket with his 1m32.974s.
This preceded a small break in running, before the field returned to the circuit on the soft compound to simulate their qualifying runs as track conditions cooled further.
Sainz was ahead of Leclerc on the road and had to endure his opening pair of sectors being narrowly shaded by the Monegasque by a whisker, but set the benchmark of a 1m32.120s regardless.
Leclerc then suffered a snap at Turns 18 and 19 and rattled over the kerb moments before crossing the start/finish line, which cost him the small margin he'd built and left him to fall short by 0.018s.
Source: Autosport