Mercedes' Valtteri Bottas beat team-mate Lewis Hamilton to qualify fastest for Saturday's Italian Grand Prix 'sprint' race.
The Finn beat Hamilton by 0.096 seconds, with Hamilton's title rival Max Verstappen third, 0.411secs behind.
The usual second Friday practice was replaced by an hour-long qualifying session to decide the grid for Saturday's 'sprint' race qualifying.
Hamilton trails Red Bull's Verstappen by three points in the title race.
Bottas, however, will take a power unit change this weekend and as a result start the main race on Sunday from the back of the grid, regardless of where he finishes on Saturday.
His lap comes days after it was announced he will be replaced at Mercedes next season by Briton George Russell. Bottas will drive for Alfa Romeo in 2022.
Bottas shouted over team radio after he crossed the line: "Oh yes - boom! Thanks."
He later added: "It feels so good when you get a nice lap. I had a bit of a tow. I feel good and relaxed now everything is sorted for the future."
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Britain's Lando Norris was an impressive fourth for McLaren with a much-improved performance from his team-mate Daniel Ricciardo seeing him take fifth.
Along with McLaren, Alpha Tauri also look quick in the hands of last year's surprise winner Pierre Gasly, who was sixth.
Ferrari's Carlos Sainz and Charles Leclerc were seventh and eighth, as Red Bull's Sergio Perez and Alfa's Antonio Giovinazzi rounded out the top 10.
Saturday's 'sprint' race results will set the grid for the main grand prix on Sunday.
There will also be championship points for the 18-lap event, with three 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 for the traditional race of 53 laps.
It is the second race of the season which has seen a change to the traditional format for grand prix weekends.
How did Bottas pull that out of the bag?
Hamilton had been fastest throughout Friday and appeared to be a country mile ahead of Verstappen and his team-mate on a circuit where he has won five times.
But, following the pain of rejection from Mercedes for next season and the announcement he will race for a new team, Bottas cut a more relaxed figure.
His first flying lap in the final session of qualifying was messy, dipping a rear wheel into the gravel at the Variante della Roggia chicane, sending stones and sparks flying.
But on the final lap he managed to take a tow and beat Hamilton, who was behind him on the road.
Hamilton said afterwards: "Congrats to Valtteri - he did a mega lap. It was looking pretty good until then, then he went quicker and I couldn't match it - well deserved.
"It's great to see Valtteri driving so well - particularly with the news this week."
Russell's reward at busy Monza
Williams' George Russell was 15th, still on a high following the announcement he will partner Hamilton at Mercedes next season.
Practice and qualifying at Autodromo Nazionale 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.
The fast low-downforce circuit is believed to suit the characteristics of the Mercedes car slightly better than the Red Bull, which has been superior at most other tracks across this season.
While some drivers benefited from gaining lap time by receiving a tow - where a car travels closely behind another at speed in clear air - there was less of a battle to follow other cars than in recent seasons.
There was close running in the pit lane during the second session, however, as cars jockeyed for position to get a gap on track, causing a near-pile-up. Both Aston Martin and Alpine were subsequently fined £4,300 each.
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