Manchester United scored twice in the final three minutes to deepen the crisis at Premier League champions Manchester City, securing a 2-1 victory at the Etihad on Sunday.
City have now won just once in their last 11 matches, a run that includes eight defeats.
Josko Gvardiol’s first-half header had given Pep Guardiola’s side the lead.
However, United responded with a win that marked Ruben Amorim’s second in five league games since taking over last month.
Bruno Fernandes’ penalty equalized, before Amad Diallo netted the winner in the final minute, squeezing a shot home from a tight angle.
The defeat leaves City in fifth, nine points behind leaders Liverpool, who have a game in hand.
United rise to 12th, now just five points behind their city rivals.
The Red Devils will hope this dramatic victory sparks a turnaround in their rivalry, which City have dominated over the past decade.
Amorim made a bold decision before kick-off, leaving both Marcus Rashford and Alejandro Garnacho out of the squad, claiming he “pays attention to everything.”
The United manager may have regretted the choice as early as the 12th minute when Mason Mount limped off, continuing his injury-plagued spell since joining from Chelsea 18 months ago.
Both teams showed a lack of confidence in a cagey opening, with Phil Foden having the first shot on goal in the 20th minute, a volley that went wide of Andre Onana’s goal.
United’s poor start to life under new management was compounded by set-piece defending, as for the third consecutive Premier League game, they conceded from a corner. Kevin De Bruyne’s deflected cross found Gvardiol, who headed in on 36 minutes.
The derby came to life moments later when Kyle Walker and Rasmus Hojlund clashed, earning both players yellow cards.
Foden had scored a double to help City win this fixture last season on their way to an unprecedented fourth consecutive league title.