Red Bull's Max Verstappen will start the Italian Grand Prix from pole with title rival Lewis Hamilton only fourth.
Hamilton's Mercedes team-mate Valtteri Bottas won the 'sprint' qualifying race but will start last after taking a penalty for a power unit change.
Hamilton finished behind McLaren's Daniel Ricciardo and Lando Norris after an uncharacteristically poor start.
Verstappen now leads Hamilton by five points in the title race after taking two points for second in the sprint.
Bottas took three points for first and Ricciardo one for third.
The usual qualifying was replaced by a half-hour, 18-lap race to decide the grid for Sunday's 53-lap grand prix, and produced drama right at the beginning.
Alpha Tauri's Pierre Gasly - the winner at Monza last year - collided with Ricciardo at the first Rettifilo chicane which eventually sent him careering into the barrier after his damaged front wing failed.
The Frenchman did a great job to minimise a high-speed impact by turning a badly damaged car away from the barrier as much as possible as it skipped across the gravel at more than 100mph.
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Verstappen said: "The race was better than expected - we had good strategy and scored nice little points and I'm starting on pole for tomorrow.
"I'm going to give it a try and stay close. We did trim our car to have decent top speed, so I'm not worried about out top speed in the race, but Mercedes have a very good pace in the whole lap, so I'll try."
Behind Hamilton were the Ferraris of Charles Leclerc and Carlos Sainz in sixth and seventh, with Alfa Romeo's Antonio Giovinazzi a creditable eighth.
Sainz, in particular, will be relived after crashing heavily in morning practice and requiring a medical check, and Leclerc recovered from feeling unwell and having to curtail his practice session.
Red Bull's Sergio Perez and Aston Martin's Lance Stroll rounded out the top 10.
Britain's George Russell, still on a high following the announcement he will partner Hamilton at Mercedes next season, will start 15th - but unusually lost out to team-mate Nicholas Latifi.
It's the second 'sprint' qualifying in a new format tweak F1 has introduced to some race weekends this season - the first being at the British Grand Prix in July.
The event at Monza took place in warm, dry conditions in Italy's Lombardy region near Milan at a track affectionately known as the 'Cathedral of Speed', owing to its rich motorsport history and old, disused banked corners nestled amid the trees in Monza's royal park.
What happened to Hamilton?
The seven-time champion had a terrible start, his car swamped by Verstappen and the McLarens by the time they reached the first chicane.
He then lost out to Gasly as he was squeezed into Rettifilo with Norris on his right-hand side before the Frenchman came to grief as he negotiated the Curva Grande.
Hamilton cut a dejected figure afterwards, blaming a poor launch away from the line.
He told Sky Sports that Mercedes have "got to try to figure out how to get by the McLarens tomorrow and limit the damage".
He added: "You saw the pace of Red Bull - I don't know if they are quicker than Valtteri, but it should be easy win for [Verstappen]. They've got more pace, and from what I understand they bring upgrades to every race."
The fast, low-downforce Autodromo di Monza is believed to suit the characteristics of the Mercedes car slightly better than the Red Bull, which has been superior at most other tracks this season.
Of the top 10 only Stroll and the McLarens used the faster soft tyre, with the others opting for the medium compound, which tyre supplier Pirelli believe is around 0.4secs a lap slower.
But there did not appear to be too much of a difference between the two as the McLarens did not suffer a loss of pace through tyre wear towards the end, nor did they have an impact on Verstappen or Bottas once positions were established early on.
Teams are free to start on any compound for Sunday's race.
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