Honda Formula 1 junior Nobuharu Matsushita became the first driver to beat Charles Leclerc to a qualifying pole in Formula 2 in 2017 with the fastest time at Monza.
ART Grand Prix driver Matsushita narrowly beat McLaren junior Nyck de Vries to the best time in the half-hour session, with Louis Deletraz completing the top three.
Championship leader Leclerc could only manage seventh place in his Prema Racing-run car, with his title rival Oliver Rowland down in 13th for DAMS.
It will mark only the second time this season Monegasque Leclerc has not started the feature race from pole, after he was excluded from qualifying at the Hungaroring in July having set the fastest time.
Matsushita moved to the top of the timesheets early on in the session, demoting early pacesetters Alexander Albon and Leclerc with a 1m30.982s that would remain the day's quickest time.
The Japanese racer, who had what he described as the "biggest crash of my life" at the previous round in Belgium, was the only driver to dip below the 1m31s barrier.
Leclerc locked up under braking for the Roggia chicane, jinking right as he did so and briefly putting his front-right wheel on the grass, and was forced to abandon his first attempt to improve his time.
That left the Ferrari F1 junior behind de Vries, Deletraz and practice pacesetter Artem Markelov during the mid-session break in action.
Shortly after the pack emerged for their second set of runs, Trident's Nabil Jeffri spun exiting the Rettifilo chicane and could not get going again, which brought out the session's first red flag.
After a short delay, the drivers jostled for position before making one final effort on the soft Pirelli rubber.
Leclerc took two laps to warm up his tyres - a tactic employed throughout the field - but could not improve his time.
He put all four wheels over the white line at the exit of the Parabolica as he surged to the line and was audibly frustrated on the radio afterwards.
Only a few drivers - notably ex-F1 driver Roberto Merhi and Jordan King - improved their positions in the final stages of the session, which was brought to a premature end when Santino Ferrucci stopped his Trident-run car on the approach to the first Lesmo corner when his rear suspension failed.
That left Matsushita on pole - his first in GP2/F2 - with de Vries just 0.089 seconds behind for Racing Engineering.
Markelov stayed fourth behind Deletraz, with Merhi and King slotting into fifth and sixth respectively.
Leclerc was stranded in seventh ahead of his team-mate Antonio Fuoco.
Ferrucci and Albon rounded out the top 10, ahead of Luca Ghiotto and Spa sprint race winner Sergio Sette Camara.
Rowland clattered over the kerbs at the Rettifilo early in the session - briefly getting airborne at the second apex - and complained of "the worst car I've driven" as he struggled with understeer around the Monza circuit.
The Renault F1 junior's team-mate Nicholas Latifi was 14th ahead of Arden International's Norman Nato.
Jeffri brought up the rear of the field, which was covered by just 1.3s.
|1||Nobuharu Matsushita||ART Grand Prix||1m30.982s||-|
|2||Nyck de Vries||Racing Engineering||1m31.071s||0.089s|
|4||Artem Markelov||RUSSIAN TIME||1m31.266s||0.284s|
|6||Jordan King||MP Motorsport||1m31.355s||0.373s|
|7||Charles Leclerc||Prema Racing||1m31.378s||0.396s|
|8||Antonio Fuoco||Prema Racing||1m31.426s||0.444s|
|10||Alexander Albon||ART Grand Prix||1m31.511s||0.529s|
|11||Luca Ghiotto||RUSSIAN TIME||1m31.529s||0.547s|
|12||Sergio Sette Camara||MP Motorsport||1m31.653s||0.671s|
|15||Norman Nato||Pertamina Arden||1m31.772s||0.790s|
|16||Ralph Boschung||Campos Racing||1m31.878s||0.896s|
|17||Gustav Malja||Racing Engineering||1m31.942s||0.960s|
|18||Sean Gelael||Pertamina Arden||1m32.171s||1.189s|
|19||Robert Visoiu||Campos Racing||1m32.267s||1.285s|