Manchester United's defensive deficiencies gifted Pep Guardiola's Manchester City a 2-1 derby victory on Sunday that allowed them to establish a massive 11-point lead in the Premier League.
Romelu Lukaku was United's fall guy, his failure to clear two set-pieces enabling David Silva and Nicolas Otamendi to score either side of Marcus Rashford's opportunistic equaliser in first-half injury time.
City's win at a snowy Old Trafford was their 14th in succession in the Premier League, equalling the English top-flight record and setting a new benchmark for consecutive victories within the same campaign.
It also gave City an enormous advantage at the summit and with Chelsea, Liverpool and Arsenal having all lost ground this weekend, it will take a calamity to deprive Guardiola's team of the title.
Guardiola has beaten opposite number Jose Mourinho in nine of their 20 meetings and this latest win came with the added satisfaction of denying United a club-record 41st home game without defeat.
City were also the last team to emerge victorious at Old Trafford, winning 2-1 in Guardiola's first Manchester derby in September 2016.
United saw a run of four wins come to an end and having lost eight of the last 13 derbies, they will find it harder to play down talk of a Manchester power shift.
In switching to a back four and naming a fleet-footed attacking quartet of Rashford, Lukaku, Jesse Lingard and Anthony Martial, Mourinho made it clear he would look to hit City on the break.
The visitors were allowed to make the running and their crisp exchanges gave United moments of alarm on the edge of their own penalty area.
Raheem Sterling created a shooting chance after a neat-one two with Gabriel Jesus and Jesus procured a sight of goal by sending Marcos Rojo crashing to the turf, but both players shot straight at David de Gea.
City made De Gea work hard for the first time in the 42nd minute when Leroy Sane controlled Fabian Delph's deep cross on his thigh and obliged the Spaniard to tip his rasping drive over the bar.
From the corner that followed, City took the lead.
Kevin De Bruyne's ball from the City right struck Lukaku and skipped up for Silva to leap and hook a close-range volley past De Gea.
City's fans were goading United, mocking cries of "Hoof!" emanating from the away end every time the hosts launched the ball forwards, but it was from one such agricultural attack that Mourinho's men drew level.
After Otamendi mistimed his attempt to head away a hopeful high ball from his Argentina team-mate Rojo, Delph allowed the ball to skip past him and Rashford darted in to clip a low shot past Ederson.
Rojo departed at half-time, having been involved in a clash of heads with Silva, while Ilkay Gundogan replaced City skipper Vincent Kompany, with Fernandinho dropping to centre-back.
United looked more enterprising early in the second half, but another set-piece in the 54th minute -- and another Lukaku blunder -- would prove their undoing.
In attempting to clear Silva's free-kick from the City left, Lukaku succeeded only in swiping it against Chris Smalling and Otamendi followed Silva's first-half lead by beating De Gea with an acrobatic volley.