Not even a goal double from top scorer Jason Scotland could save the Swans from crashing to their heaviest defeat of the season at Northampton Town.
The Swans suffered a nightmare in the first 23 minutes at Sixfields as the Cobblers stormed into a 3-0 lead to maintain their play-off challenge.
Two goals from Poul Hubertz, an own goal from Alan Tate and a fourth from Danny Jackman gave Northampton the points before Scotland hit back.
Despite the defeat, the Swans remain six points clear at the top after Doncaster lost and Carlisle drew.
NorthamptonTown: Bunn, Crowe, Gyepes, Little, Jones, Gilligan, Burnell, Jackman, Holt, Hubertz, Akinfenwa (Coke 73). Subs not used: Dunn, Henderson, Dolman, Dyer.
Booked: Crowe.
SwanseaCity: De Vries, Rangel, Tate, Lawrence, Painter (Pratley 46), Butler, Bodde, Britton, Robinson, Bauza (Brandy 46), Scotland. Subs not used: Austin, Duffy, MacDonald.
Att: 5,926 Away: 858
Ref: Jon Moss (W Yorkshire).
The Swans made two changes to the side that drew 1-1 with Tranmere in midweek as Marcos Painter returned at left-back following a hamstring injury. He replaced Kevin Austin who had to be content with a place on the bench.
Striker Guillem Bauza was also given a start in place of midfielder Darren Pratley as manager Roberto Martinez switched from a 4-5-1 formation to a more orthodox 4-4-2.
Skipper Garry Monk remained sidelined with a back injury.
They faced a Northampton side in a fine run of form and pushing hard for a place in the play-offs.
They were led up-front by former Swans striker Bayo Akinfenwa who returned from a virus having scored six goals in as many games.
And Akinfenwa gave the Swans an early warning as he thundered a Danny Jackman cross against the bar with a great header.
It was a warning Swansea failed to take as they fell behind after just 14 minutes when Akinfenwa headed down a corner into the path of Poul Hubertz to put the home side ahead from six yards.
Five minutes later and Northampton doubled their lead when Daniel Jones' cross was deflected passed Dorus de Vries vi the crossbar by Swans defender Alan Tate for an own goal.
And it went from bad to worse for the Swans. With just 23 minutes gone they were 3-0 down when Ferrie Bodde brought down Ryan Gilligan inside the area and referee Jon Moss had no hesitation in pointing to the spot.
Hubertz beat de Vries at his bottom corner and the Swans looked dead and buried.
Jason Scotland did give Swansea a lifeline in the 29th minute when he got in behind the Cobblers defence to latch on to a Bodde pass.
Although Mark Bunn saved his first effort, Scotland tapped home the rebound for his 23rd goal of the season.
Scotland had a great chance to reduce the deficit even further soon after when Jason Crowe's slip allowed Andy Robinson to put him through. But Bunn did well to push the striker wide before Bauza fired wide.
Half-time: Northampton 3 Swans 1
Darren Pratley came on for Painter at the start of the second-half as Swansea went for broke with three at the back.
And Scotland had another great chance to grab his second straight after the re-start when Bunn sliced his kick straight at him. But the striker was unable to control the ball and the chance went begging.
It proved costly as Northampton grabbed a fourth in the 52nd minute after dreadful defending allowed Akinfenwa to find Danny Jackman who fired into the far corner.
Akinfenwa nearly increased that lead when he sent a free header wide from eight yards, before Swansea snatched a consolation effort seven minutes from time.
Bodde was again the creater when he played Scotland through and the Trinidad & Tobago striker grabbed his second of the game to take his tally for the season to 24.
It was the only plus point in a disappointinmg afternoon.
