At the start of the one-hour session, Nyck de Vries led the pack out of the pits and ran a large aerodynamic measuring device behind the front wheels of Sebastian Vettel’s Aston Martin.
When the 2021 Formula E champion struck before setting a timed lap – which he wouldn’t do until close to halfway as Aston concentrated early on its aeroload data collection – Haas’ Kevin Magnussen set the first place benchmark at 1m27.185s .
This was quickly beaten by a series of faster times, with Sainz, Esteban Ocon and Sergio Perez all enjoying brief stints in the top spot as the pack ran a variety of tire compounds during the opening race.
At the end of the opening races, Verstappen moved into first place for the first time with a 1m25.830s set on the hard tyres.
Perez then climbed back with a 1m25.026s, also on the white-walled rubber, before Fernando Alonso moved into first next time thanks to his 1m24.935s on the hards as well.
Verstappen’s opening run on his first hard set continued and he finished the first 10 minutes in first place thanks to his 1m23.449s, which he beat twice more before there was a lull in activity ahead of the opening qualifying simulations – the benchmark down to a 1m22.853s .
Yuki Tsunoda had been the first driver to complete a flier on the softs before the break around the 20-minute mark, but finished 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 out on the softs and he immediately took top spot from Verstappen with a 1m22.410s.
The Red Bull drivers went out on softs a few minutes later, with Perez’s opening flight effort coming well over a second behind Leclerc’s best.
Verstappen, who had a slight lock up on his out-lap with his new softs at the circuit’s first chicane, looked set to beat Leclerc’s time before encountering traffic in the final sector.
Lando Norris was over four seconds off the pace still on the hard tires when Verstappen caught him just before the Ascari chicane and the Briton didn’t budge from the Red Bull’s way and led him to the finish.
There, Verstappen improved his personal best to finish second, but lost 0.7s to McLaren to go from being 0.3s up on Leclerc, before catching Norris to 0.4s behind.
The Mercedes cars were next to try the soft cars, with Russell finishing 0.5 seconds behind Leclerc in third after having to pass several slow-moving cars on the back right at the end of his lap.
Hamilton’s first run on the softs initially left him only seventh and just over a second slower than Leclerc as he had to catch big slides exiting the second chicane and midway through the Ascari complex.
Russell then completed another run on the softs to move ahead of Verstappen and close to 0.279s slower than Leclerc.
Hamilton then also moved Verstappen down by 1m22.831s after Sainz had set a time just 0.077s slower than his team-mate.
Sainz had run an offset plan to Leclerc after Ferrari had to work on his F1-75 in the pits after the opening laps, with the Spaniard sent out again for a high-fuel run on the hards while Leclerc put his flying shot at the soft ones.
But Sainz’s 1m22.487s almost matched Leclerc’s time as he was finally able to complete an initial qualifying simulation just before the final quarter of the session started.
Verstappen again went on the soft tracks in the closing stages but only completed high fuel running well off the pace and his fifth place result was cemented.
Ocon finished sixth ahead of Tsunoda, whose subsequent later run on the softs meant he closed to 0.850s ahead of Leclerc.
Valtteri Bottas and Daniel Ricciardo rounded out the top 10, with Norris down to 16th in the second McLaren.
De Vries finished with the 19th fastest time ahead of Williams driver Nicholas Latifi, with the Aston rookie having a short off through the Ascari gravel during a late flyover.
Several teams, including Red Bull, ran different rear wing specifications as they began to find the best set-up for Monza’s long straights.
Perez’s rear wing was also seen oscillating wildly as he raced down the main line during the early laps with Red Bull working on the problem, which also appeared in the early rounds of the season during the rest of the session.