It is just seven months and one day since Michael Laudrup's side lifted the first major honour in their 101-year history with a thumping 5-0 victory over Bradford at Wembley.
But the holders were brought crashing back down to earth by another side who also share fond recent memories of the competition - 2011 winners Birmingham.
It was a classic game of two halves in the third-round tie as the Swans saw plenty of the ball in the opening period and hit the woodwork twice within a matter of seconds through Wilfried Bony and Alejandro Pozuelo.
But the Sky Bet Championship hosts appeared a different animal after the break and Dan Burn headed home his first goal for the club in the 56th minute before Matt Green added a second from close range soon after.
Inspired by winger Chris Burke, who played his part in all three goals, Tom Adeyemi also got in on the act with his maiden goal late on - with Bony netting a consolation with virtually the last touch.
Laudrup had made no secret of his desire to lift the League Cup in successive seasons, despite the heavy demands of the Europa League, although he did make 10 changes from Sunday's Barclays Premier League victory at Crystal Palace, with midfielder Jonjo Shelvey the only survivor.
Yet his in-form side, having won three of their last unbeaten four including a memorable 3-0 triumph in Valencia, were unable to reproduce their heroics from the Mestalla in the midlands.
The only downside of the evening for Birmingham was that there was only 7,470 in attendance to watch what was arguably the biggest upset of the third round.
There was very little for the sparse crowd to get excited about inside the opening stages, but that all changed in the 18th minute.
Jonathan de Guzman's corner was headed goalwards by Bony but Paul Caddis, stood next to the back post, reacted quickly to nod the ball up against the crossbar and out in the direction of Pozuelo, who duly hammered the rebound back against the woodwork.
Yet no sooner had the excitement passed than the game fell back into its previous pattern - Swans passing the ball around neatly without great penetration and Blues keen to hustle and counter where possible.
Bony was proving the liveliest outlet on the pitch and he saw a close-range shot well blocked in the 30th minute before curling another effort narrowly wide soon after.
Birmingham skipper Paul Robinson produced a great tackle to deny the Ivorian when he found himself one-on-one before goalkeeper Colin Doyle used his legs to block from Roland Lamah when he was also sent clean through.
The hosts immediately looked more dangerous after the interval and Green fired their first real chance straight at Swans stopper Gerhard Tremmel, inspiring the home crowd in attendance.
And it was the hosts who duly broke the deadlock - Burke finding Caddis down the right and his cross was headed home by Burn from six yards.
Just five minutes later and Birmingham were in dreamland as Burke, again the creator, found Mitch Hancox down the left and he crossed low for Green to fire home from close range.
Swansea were reduced to long-range efforts as they searched for a route back into the game.
But all hopes of a comeback were extinguished in the 81st minute.
Adeyemi caught out Jordi Amat on the edge of his own penalty area and the ball broke for Burke, who played the ball back to the midfielder to wrap up a superb evening for Lee Clark's side - despite Bony's injury-time header from Wayne Routledge's corner.