Mercedes' Lewis Hamilton was fastest in first practice for the Italian Grand Prix ahead of title rival Max Verstappen of Red Bull.
Hamilton, running medium tyres, was 0.452 seconds ahead of Verstappen on the soft compound.
Valtteri Bottas was third, 0.525secs down on team-mate Hamilton.
The usual second practice will be replaced by an hour-long qualifying session to decide the grid for Saturday's 'sprint race' qualifying.
It is the second race this season in which an F1 weekend's format has been tweaked, after July's British Grand Prix.
Aston Martin's Lance Stroll was a surprise fourth fastest, with team-mate Sebastian Vettel sixth. Both Aston Martin and Alpha Tauri are midfield teams who appear to have good pace at the low-downforce Monza circuit.
Alpha Tauri's Pierre Gasly - last year's shock race winner - was fifth, with Carlos Sainz seventh for Ferrari, Fernando Alonso eighth for Alpine, Daniel Ricciardo ninth for McLaren, and Verstappen's team-mate Sergio Perez rounding out the top 10.
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Saturday's 'sprint' race will set the grid for the main grand prix on Sunday.
There will be championship points for the 'sprint' event, with three going to the winner, two for second and one for third.
The winner of the sprint race will be officially recognised as the driver who scored pole position.
Hamilton currently trails Verstappen by three points in the title race.
First practice at Monza took place in warm, dry conditions in Italy's Lombardy region near Milan. The track is known as the 'cathedral of speed', owing to its rich motorsport history.
Williams' George Russell was 17th fastest, still on a high following the announcement he will partner Hamilton at Mercedes next season.
Bottas will drive for Alfa Romeo in place of Kimi Raikkonen in 2022.
Raikkonen will miss a second race in succession following his positive coronavirus test before qualifying at the Dutch Grand Prix last week.
Veteran pole Robert Kubica, who will once again replace the Finn, was just ahead of Russell in 16th.
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