Manuel Locatelli's stunning strike earned AC Milan a 1-0 victory over Serie A leaders Juventus, who controversially had a Miralem Pjanic goal ruled out at San Siro.
Teenage midfielder Locatelli fired a sensational effort beyond Juve goalkeeper Gianluigi Buffon in the 65th minute, with Vincenzo Montella's side consequently moving to within two points of their table-topping opponents on his 200th Serie A match as a head coach.
The champions may feel hard done by, though, with Pjanic seeing a goal chalked off in the first half due to a debatable offside decision against Medhi Benatia.
After losing Paulo Dybala to injury, the midfielder's 35-yard free-kick flew beyond the outstretched leg of Leonardo Bonucci, who raced in from a legal position, to nestle in the back of the net.
The goal initially stood, but referee Nicola Rizzoli consulted with his assistants following protests by the Milan players and pulled play back for Benatia's infringement, despite him not appearing to have interfered with play.
It was a bitter defeat for Massimiliano Allegri's side, whose only prior loss this term came in the same stadium when they were beaten 2-1 by Inter.
It took Juve less than a minute to test Gianluigi Donnarumma, but Dybala's attempt to guide Alex Sandro's cross inside the near post was held by the goalkeeper.
The Turin giants looked in control but received a warning when Carlos Bacca, who replaced Gianluca Lapadula in the side after being rested for the 3-1 win over Chievo last weekend, fed Suso, whose curling attempt was pushed behind by Gianluigi Buffon.
Dybala forced another save from Donnarumma before being forced off in the 33rd minute after appearing to tweak his right hamstring while breaking forward and attempting an audacious long-range strike, with Juan Cuadrado replacing him.
Pjanic thought he had given the visitors the lead three minutes later when his 35-yard free-kick flew into the back of the net, but after the visitors had celebrated the goal, it was then overturned due to the offside controversy.
Juve returned from half-time motivated to break the deadlock and Gonzalo Higuain fired wide from the right-hand side of the box.
Their positive impetus soon subsided, though, with Milan punishing them midway through the second half. Locatelli collected Suso's pass and took one touch on the right-hand side of the box before rifling a shot across Buffon and in off the bar.
Allegri's team were unable to create any clear opportunities as they attempted to restore parity, with Higuain receiving minimal service in a laboured performance in front of goal.
The introduction of Mario Mandzukic, back from an adductor injury, with 15 minutes remaining failed to galvanise the visitors further.
But Donnarumma did have to produce a stunning save to keep a 25-yard strike from Sami Khedira out of the top corner in the fifth minute of stoppage time, as Juve finally registered their first shot on target of the second half.
That meant Milan's brilliant start to the campaign continued with a first top-flight win over Juve since November 2012 - ending a run of seven consecutive top-flight defeats in the fixture.