Manchester United fell behind early but struck four times to top rival Man City in Sunday's derby.Manchester United beat city rival Manchester City for the first time in five Premier League attempts with a stirring 4-2 comeback victory at Old Trafford.
City's title defense had crumbled heading into the match amid a run of five defeats in seven matches across all competitions, but the champion briefly displayed its fluent best in the early stages - Sergio Aguero's seventh goal in derbies after seven minutes the reward.
The direct, high-tempo approach United has found success with in recent weeks provided the route back into the match, with Ashley Young leveling before supplying the ammunition for Marouane Fellaini to give the home side a 27th-minute lead.
Although Louis van Gaal's men were fortunate to be ahead at that stage, they proceeded to dominate their neighbor, which slipped into the kind of lethargic performance now synonymous with its 2015 efforts - heaping pressure on Manuel Pellegrini.
Juan Mata, the hero of United's recent triumph at Anfield, added a third midway through the second half amid strong suspicions of offside and Chris Smalling put the game beyond doubt, with Aguero's late strike - his 100th for City - providing scant consolation for the visiting club.
United sits four points clear of fourth-placed City, which faces a battle for UEFA Champions League qualification with the season on the brink of total collapse.
Smalling - sent off in United's 1-0 loss at the Etihad Stadium in November - returned from illness to line up in the home defense, with Marcos Rojo dropping to the bench.
City's back line was boosted by captain Vincent Kompany passing a late fitness test.
Despite the wretched recent form, City settled quickly and Jesus Navas raced clear of the United defense on the end of Martin Demichelis' lofted pass, but fellow Spain international David De Gea stood firm to deny him.
Pellegrini's team pegged its opponent back thereafter and, in a move that echoed the duo's combination play in City's famous 6-1 win at Old Trafford in October 2011, James Milner's reverse pass picked out David Silva to set up Aguero for a simple finish.
The visiting side pressed forward to increase the damage, but United was back on terms after 14 minutes, Young scrambling home Ander Herrera's low cross at the second attempt.
Young turned then turned provider, combining cleverly with Daley Blind on the left flank and hanging up a cross for Fellaini to power past Joe Hart at the back post.
Fellaini's muscular presence caused City a persistent problem and Pablo Zabaleta made a vital headed clearance to deny the Belgium international a second in first-half stoppage time.
Kompany - who was booked for a rash lunge on Blind late in the first half - made way at the break due to further injury problems, meaning a first derby appearance for Eliaquim Mangala.
United came close to a third in the 52nd minute as Hart pushed a Wayne Rooney free kick out of the top corner and the England goalkeeper saved from Michael Carrick's follow-up amid a frantic penalty box scramble.
City's early poise had faded to a distant memory by the time United made the points safe in the 67th minute - Demichelis out of position as he ceded possession to Blind, who found Rooney to send Mata scampering through for his eighth league goal of the season.
Questions over Pellegrini's position in the Etihad Stadium dugout are now sure to increase, and the scrambled mindset of his players was thoroughly exposed when Smalling was granted ample room to head home Rooney's free-kick amid further Old Trafford delirium.
Aguero then tucked home Zabaleta's 89th-minute cutback to reach his milestone on arguably the bleakest day of his City career.