This had the ominous look of a defining moment in Manchester City's season. After losing their discipline off the field and three crucial points on it, their title destiny is no longer in their own hands.
Roberto Mancini's side appeared jaded after two games in a week. Their Europa League defeat in Lisbon on Thursday evening highlighted a lack of finesse in front of goal, and on this occasion they had 18 shots on target and 13 more that finished either high or wide of Swansea goalkeeper Michel Vorm's goal.
Little wonder the Italian manager spent much of his afternoon pacing up and down the technical area, remonstrating with his players.
The only visitor who appeared more exercised than Mancini was Gareth Barry, whose first-half substitution prompted a furious outburst at the Italian's assistant, David Platt. The latter shrugged his shoulders while Barry threw down his boots in disgust.
It was that kind of day for the title pretenders and this result will rank alongside Blackburn's victory at Old Trafford on New Year's Eve in this season's file of unlikely triumphs.
Swansea, who began the season as favourites to suffer the indignity of relegation and whose side cost a mere £8million to build, beating Manchester City beggared belief.
However, for the 20,510 who crammed into the Liberty Stadium, it was anything but a surprise. Although City's second-half recovery left them with an equal amount of possession and a greater percentage of the overall shots on target, it was Swansea who had a sixth-minute penalty saved.
Swansea boss Brendan Rodgers can claim to have been in charge of the better side. They dominated for 36 minutes of the first half, in terms of territory and possession, and showed a resilience that will ultimately keep them at the top table for another season at least.
Luke Moore's goal, from a pin-point cross from Wayne Routledge, was no more than they deserved.
The signs had been clear for City from the first minute when Scott Sinclair set off along the left and teased Micah Richards into virtual submission. Fortunately for the visitors, nothing came of that foray as Routledge blazed over at the far post. But within five minutes, City could easily have gone behind when goalkeeper Joe Hart clipped the heels of Routledge as he burst onto Sinclair's delightful through ball.
Referee Lee Mason pointed to the spot, but Sinclair, opting for Hart's right-hand corner, was careless in his execution. The England goalkeeper stretched out a hand and beat the ball to safety.
It should have been the wake-up call to spur Mancini's side. But hustled off the ball in midfield, they barely had a kick until the final nine minutes of the half when a succession of corners and a decent strike from Nigel De Jong provided a flickering reminder of their potential.
That spell coincided with Mancini's decision to replace Barry. Having switched to 4-4-2, City stopped Swansea at source. With more ball, territory and clearly greater confidence, they began to pose serious questions of their own.
Sergio Aguero and then Samir Nasri were denied by the impeccable Vorm, while Mario Balotelli showed some useful touches before joining De Jong in the book for a rash challenge on Gylfi Sigurdsson.
Swansea survived and, with 12 minutes remaining, Rodgers replaced Danny Graham with Moore. Within five minutes, the former Aston Villa striker had claimed victory. Sigurdsson burgled possession in midfield before Routledge delivered the perfect centre for Moore to rise and head home at the far post.
City's heads dropped and with news filtering through that Manchester United were heading to victory at Old Trafford, Mancini threw on Adam Johnson and Edin Dzeko for one final push.
Micah Richards headed home Balotelli's free kick two minutes from time and peeled away to accept the plaudits of his team-mates. His joy was short-lived as assistant referee Sian Massey held her flag aloft.
Television replays confirmed that Richards was offside. It was the final straw.