Milan's unbeaten run came to a shuddering halt after 11 games as Yusuf Yazici's hat-trick secured an emphatic 3-0 victory for Lille in the Europa League.
Inspired by the form of Zlatan Ibrahimovic – who already has eight goals to his name – Stefano Pioli's side have made a wonderful start to 2020-21.
But Ibrahimovic, who was subbed off in the 62nd minute, failed to lift the Rossoneri in a dismal showing at San Siro, with Yazici running the show.
The 23-year-old Turkey international opened the scoring from the spot 21 minutes in, before becoming the first player to score a hat-trick against Milan in all competitions since Rivaldo in October 2000 with quickfire strikes early in the second half to move Lille one point clear at the top of Group H.
Ibrahimovic crafted the game's first chance, teeing up Theo Hernandez to shoot from a tight angle, with his effort deflected wide.
Yet Lille had the lead two minutes later – Yazici drilling home from the penalty spot after drawing a clumsy foul from Alessio Romagnoli.
Along with former Lille forward Rafael Leao, Hakan Calhanoglu was introduced at the break, yet it was the latter's compatriot Yazici who delivered a second blow to Milan when Gianluigi Donnarumma failed to keep out his dipping effort.
Milan were all at sea and Lille took full advantage, Yazici curling in his third with a brilliant first-time finish at the culmination of a fast break.
Pioli made the surprising decision to take Ibrahimovic off moments later, though the change nearly paid dividends as his replacement Ante Rebic drew a smart stop from Mike Maignan.
Donnarumma made slight amends for his earlier mistake with a brilliant save to deny Yazici a fourth, but the damage was already done as Lille won consecutive away games in the Europa League for the first time.