At the start of the hour-long session, Nyck de Vries led the field out of the pits, running a big aero measuring device behind the front wheels of Sebastian Vettel’s Aston Martin.
When the 2021 Formula E champion stopped short of setting a timed lap – which he would only do around halfway through the race as Aston focused on collecting aero load data early on – Kevin Haas’ Magnussen set the benchmark for first place at 1m27.185s.
That was quickly beaten by a host of faster times, with Sainz, Esteban Ocon and Sergio Perez all enjoying brief stints in the top stop as the peloton used a range of tire compounds in the initial run.
At the end of the first heats, Verstappen took first place for the first time with a time of 1m25.830s on hard tyres.
Perez then took the lead again with a 1m25.026s, also on the white-walled rubber, before Fernando Alonso took the lead the next time thanks to his 1m24.935s on the hard ones as well.
Verstappen’s opening run on his first set of hard drives continued and he finished the first 10 minutes in first place thanks to his 1m23.449s, which he beat twice more before there have a lull in activity ahead of the opening qualifying simulations – the benchmark up to a 1m22.853s.
Yuki Tsunoda had been the first driver to complete a flyer on the softs before the lull around the 20-minute mark, but found himself more than a second behind Verstappen’s best time on the hards.
When the action heated up again just before the halfway mark, Leclerc led the front-runners heading into the softs and he immediately took the lead from Verstappen with a 1m22.410s.
The Red Bull drivers came out on the softs a few minutes later, with Perez’s opening flight effort well over a second adrift of Leclerc’s best.
Verstappen, who had a small blockage on his exit lap with his new softs at the track’s first chicane, looked set to beat Leclerc’s time before encountering traffic in the final sector.
Lando Norris was more than four seconds off the pace still on hard tires when Verstappen caught him just before the Ascari chicane and the Briton didn’t move out of the Red Bull’s way and led him to the finish.
There Verstappen bettered his personal best to second but fell 0.7s past the McLaren to go 0.3s over Leclerc before catching Norris 0.4s behind.
The Mercedes cars were next to try their hand at the softs, with Russell finding himself 0.5 seconds behind Leclerc in third after having to pass several slow cars on the back straight at the end of his lap.
Hamilton’s first race on the softs initially left him only seventh and just over a second slower than Leclerc as he had to catch some big slides coming out of the second chicane and halfway through the Ascari complex.
Russell then completed a second run on the softs to edge Verstappen and nearly 0.279s slower than Leclerc.
Hamilton then demoted Verstappen as well with a 1m22.831s, after Sainz clocked just 0.077s slower than his team-mate.
Sainz had executed a plan off Leclerc after Ferrari had to work on his F1-75 in the pits after the early laps, with the Spaniard sent off for a high-fuel run on the hard-packs as Leclerc put on his tour launched on the softs.
But Sainz’s 1m22.487s nearly matched Leclerc’s time when he was finally able to complete a first qualifying simulation just before the start of the final quarter of the session.
Verstappen went back to the softs in the final stages but only finished high-fuel runs well off the pace and so his fifth-place result was cemented.
Ocon finished sixth ahead of Tsunoda, whose subsequent run on the softs saw him closing in on Leclerc at .850.
Valtteri Bottas and Daniel Ricciardo closed the top 10, with Norris in 16th in the other McLaren.
De Vries finished with the 19th fastest time ahead of Williams driver Nicholas Latifi, the Aston rookie having a brief stint through the Ascari gravel on a late flight.
Several teams, including Red Bull, applied different rear wing specs as they began to find the best setup for Monza’s long straights.
Perez’s rear wing was also seen swaying wildly as he descended the main straight during the opening laps, with Red Bull working on the issue, which also appeared in the opening motos of the season, while throughout the session.
Italian F1 GP: Ferrari leads FP1 as Leclerc overtakes Sainz