Seven months after winning the Premier League with Manchester City, Adam Johnson inflicted a blow to his former club's title defense by clinching a 1-0 victory for Sunderland on Wednesday.
The England winger, who was considered surplus to requirements at City in the offseason, embarrassed former teammate Joe Hart with a dipping strike in the 53rd minute that beat the goalkeeper at the near post.
The setback left second-place City seven points behind leader Manchester United at the season's halfway point.
"We had a lot of chances, we played well, but we didn't score," City manager Roberto Mancini said. "This is our problem, we don't score. We don't score enough goals to win the game.
"Last year, we scored one, two, three goals every game. Now when we have chances, we are too soft and this is not good. We are too soft in attack, too soft when we have chances, too soft when we shoot."
Sunderland turned in one of its best displays of the season, with Hart having to turn away Steven Fletcher's skidding effort and keep out Stephane Sessegnon late on.
But Sunderland goalkeeper Simon Mignolet was also alert to deny Yaya Toure and David Silva before the break, and Sergio Aguero in the second half as City slumped to a third consecutive 1-0 loss at Sunderland.
"They are very lucky against us," Mancini said. "They defend well, but we need to be stronger."
City might have been ahead with just eight minutes gone. Captain Vincent Kompany's header came back off the crossbar and Yaya Toure's goalbound follow-up was hacked away by Mignolet.
Silva curled a 10th-minute free-kick wastefully over the bar and Toure saw a curling effort plucked from underneath the crossbar by Mignolet three minutes later.
City was pinning Sunderland deep inside its own half and it seemed only a matter of time before they made the pressure tell.
Mignolet had to be at his best once again after Carlos Tevez and Aguero combined to put Silva in on goal, only for the goalkeeper to block his close-range effort.
Seconds later Hart had to dive to his right to turn away Fletcher's sharp left-foot drive and Sunderland, buoyed after weathering the early storm, started to force its way into the game.
Johnson was enjoying the better of his individual battle with Pablo Zabaleta, who played much of the half with a heavily bandaged head after suffering a cut, and Sunderland upped the tempo.
The hosts picked up where they had left off in the second half as James McClean and Johnson stretched City, and they soon took the lead.
After City manager Roberto Mancini was left appealing in vain for a free kick for Craig Gardner's challenge on Zabaleta, Johnson picked up possession and cut inside before blasting in a dipping shot that caught Hart flat-footed at his near post and squirmed into the bottom corner.
Mancini, who absolved Hart of blame for the goal, was angry that play was allowed to continue by referee Kevin Friend in the run-up to the goal.
"The referee ate too much for Christmas. He was not in good form," Mancini said. "He and the linesman, I think, after Christmas, it can happen to have a bad performance like today."
Sessegnon might have doubled his side's advantage soon after, but pulled his effort across the face of goal with City in disarray.
The visitors were almost breached once again on the hour when McClean picked out Sessegnon in front of goal, but the Benin international could not take the ball in his stride.
Kompany, who had already been booked, was fortunate to escape a second yellow card for a block on McClean as City struggled to contain a Black Cats side growing in confidence by the minute.
City gradually regained their composure and started to move the ball confidently once again and Mignolet had to claim a 68th-minute Zabaleta cross at his near post after being played in by James Milner.
The Belgium goalkeeper excelled once again two minutes later to deny Aguero one on one as deep Sunderland defending prevented City from finding a breakthrough.