Piastri, who leads the championship by 16 points, posted a time of 1:15.293s in warm and sunny conditions at Imola to best Norris by just 0.025s.
Both McLaren drivers went for a second run on the softest tyre compound – which for this weekend is Pirelli’s new C6 – but neither unearthed gains.
Norris, having been on course to demote Piastri, made a mistake through the final corner and abandoned his effort, while Piastri was slower all-round.
Alpine’s Pierre Gasly surprisingly emerged as the fastest non-McLaren driver, putting his A525 into third position, just over a quarter of a second behind Piastri.
George Russell was fourth overall for Mercedes, though team-mate Kimi Antonelli had a more subdued day on home soil, finishing only 18th after erring into the Gresini chicane and taking to the grassy run-off.
Max Verstappen was again unhappy with the rear of Red Bull’s RB21 en route to fifth spot, four-tenths of a second behind Piastri at a venue where he has claimed victory on Formula 1’s last three visits.
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Ferrari had a difficult session in front of its fans on home territory.
Charles Leclerc and Lewis Hamilton both complained about the SF-25’s brakes, with Leclerc calling them “horrendous” in warm-up mode, while Hamilton described them as “a problem.”
Leclerc finished the session in sixth place while Hamilton was left down in 11th position.
Isack Hadjar was seventh for Racing Bulls but caused a red flag during the closing stages of the session after spinning off through Tamburello.
Hadjar made only light contact with the barriers and was able to extricate the car through most of the gravel trap, but it eventually became beached by the side of the circuit, leaving race control with no option but to suspend the session.
Red Bull’s Yuki Tsunoda was eighth, leaving the Williams duo of Alex Albon and Carlos Sainz to round out the top 10.
Franco Colapinto finished his first full day of competitive track action for Alpine in 13th position.
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