Sergio Aguero inflicted more pain on Manchester United as Manchester City won the derby 2-1 to delay the Red Devils' title celebrations.
The man whose goal won the Premier League for Manchester City last season struck 11 minutes from time to give the Blues back-to-back derby wins at Old Trafford for the first time in 41 years. United's first defeat since November also ended its 18-match unbeaten run and cut its lead at the top of the Premier League to 12 points.
"We showed we deserved to stay closer to the title but it is finished," City manager Roberto Mancini said, maintaining his stance of the past three weeks.
Seeking a record 20th English league title, United (25-4-2) lost a league match at home for the first time since a 3-2 defeat to Tottenham on Sept. 29. Overall, United had been 16-0-2 in the league since a 1-0 defeat at Norwich on Nov. 17.
"You don't want to lose a derby game, but we're OK," United manager Alex Ferguson said. "It is easy to see why these were the best two teams in the country; the evidence was there tonight."
After James Milner's second-half opener had been cancelled out by a Vincent Kompany own goal, the result at least gives credence to Mancini's claim that United are not quite deserving of their massive advantage.
Not even Mancini truly believed United's 15-point lead could be overhauled even if his side won, so, as Kompany said last week, the prize was unofficial champions of Manchester. And City was quickest to settle, exploiting an open approach from the hosts who, including Ryan Giggs, had five forwards in its starting lineup.
City's best opportunity was denied it by a tremendous intervention from Phil Jones. Samir Nasri peeled onto Rafael and nodded Kompany's long pass down for Carlos Tevez. The one-time darling of the Old Trafford faithful was just about to pull the trigger when in nipped Jones to make a magnificent clearance.
But it was not until the dregs of that opening half, when Rafael ran onto Robin Van Persie's flick and lifted the ball onto the angle of post and bar that United properly threatened.
And within five minutes of the restart they fell behind. Giggs must shoulder the burden of responsibility for an ill-advised backheel that gifted possession to Gareth Barry deep inside United territory. The visitors' rapid response allowed them to tee up Milner, whose low drive flicked off Jones and flew into the bottom corner.
The hosts' pride demanded a quick response. And they got exactly that as Van Persie fizzed a free kick to the far post. As Joe Hart failed to make any connection, Jones got his head to the ball and although it was not directed goalwards, it struck Kompany on the back to divert it in.
Yet City was the one which responded more positively. And after Nasri had fired over, the man he replaced, Aguero, put the visitors back in front. It was eerily similar to his decisive goal last May too as the Argentina star ghosted past Jones and Rio Ferdinand before firing into the roof of David De Gea's net.
In the fractious latter stages, Barry was booked for starting a skirmish, Pablo Zabaleta's yellow-card challenge forced Ashley Young off and both Rafael and Antonio Valencia were cautioned for felling Barry. It all felt slightly hollow, though, as City's fans sang about being the "pride of Manchester" whilst knowing their title is almost gone.
Information from Press Association and The Associated Press contributed to this report.