Amid a series of qualifying simulations over the second half of the session as the sun began to fall, Ferrari elected to wait for the track temperature to drop a little more before sending both drivers out for their first attempts at an all-or-nothing flying lap.
Sainz's 1m32.065s on his first push-lap on the soft tyres awarded him with top spot, and the Spaniard's time managed to withstand a late siege amid a flurry of qualifying simulations.
Sainz had earlier led the line before being displaced by Russell, as the Mercedes duo sat atop the order after the opening half-hour of running. Russell had been leading Hamilton as the two had eschewed the medium compound used by their front-running rivals for an early series of soft-tyre laps.
Russell's 1m32.364s on the soft had vaulted him back to the top, but was beaten by Lando Norris' 1m32.303s. The McLaren driver's time could not hold firm from Sainz's lap, as the two Ferraris took to the stage.
Although the Ferrari pair ran the risk of encountering a sector three yellow flag when Alex Albon went off at Turn 17, the green flag was waved at the chicane just in time for them to pass through unrestricted.
On a second tilt, Russell leapfrogged Norris to reclaim second while Sainz attempted to better his time on a second run - but the Monza polesitter instead found himself performing a 50-50 grind on the Turn 7 exit kerb and had to abort his run. Team-mate Charles Leclerc also had to back out of a lap after setting the fastest first sector, which denied Ferrari the chance of securing another practice 1-2.
Source: Autosport