Nottingham Forest were on the cusp of surviving a late scare before Swansea City's two late goals sprang a major upset.
Igor Jesus scored an impressive brace for the visitors before Cameron Burgess' header pegged them back. Forest were flowing in possession and creating several chances, but were unable to convert them, which ultimately cost them dearly.
Substitute Zan Vipotnik's flicked effort levelled the tie, and centre-back Burgess matched the feat of Jesus with a brace to send the Swansea faithful into raptures.
Story of the Match
Ange Postecoglou made ten changes from the defeat at Arsenal, handing starts to eight of the summer signings, includingDouglas Luiz. The Brazilian had the chance to get his first minutes in a Forest shirt under his belt after joining on loan from Juventus.
Several stalwart names from last season were absent from the matchday squad, but the strong squad on paper highlighted Postecoglou's array of options in his ranks since inheriting Nuno's post.
Swansea City manager Alan Sheehan offered youngsters Ishe Samuels-Smith and Manuel Benson the opportunity to impress against Premier League opposition on their debuts after being acquired on loan from Chelsea and Burnley, respectively.
The hosts were unbeaten this campaign heading into their third round tie in Wales and were looking to progress to the fourth round for the first time in eight years.
Swansea set the tone early amidst the challenging conditions and were ultimately striving to thwart Nottingham Forest from seizing an immediate advantage and dispatching them comfortably.
However, Omari Hutchinson had already isolated Josh Key on the left-wing and looked poised to bypass his marker with more intense directness.
Dilane Bakwa sprayed the ball out to Forest's record signing, who edged into the penalty area before the ball dropped to Igor Jesus to slot it beyond Andrew Fisher.
The Brazilian breakout star from the Club World Cup opened his account for Forest and initiated the Forest move by dropping deeper.
Swansea attempted to mount a response and enjoyed the lionshare of possession for a considerable period.
Ji-Sung Eom proved a bright spark in attack and was afforded space in the pocket to test John Victor from range, yet his effort strayed off target.
Forest were unable to gain a foothold of the game and Swansea mitigated their hopes of securing a cushion.
Penalty appeals were even waved away when Malick Junior Yalcouye was stopped in his tracks by Willy Bolly.
After a spell of little action in the final third for the visitors, Bakwa teased and shrugged off Samuels-Smith, gliding down the right. He fired his pass into James McAtee, who was unable to extend the advantage.
The French winger, who signed from Strasbourg in the summer, was in the thick of the action again when he instigated a well-crafted attack by Forest.
Nicolo Savona was unattended on the right before McAtee's inch-perfect ball to Hutchinson across the six-yard box spelt serious danger. Hutchinson teed up Jesus to seal his brace with a simple tap-in at the stroke of half-time.
The Championship outfit would have been dejected to concede another while Postecoglou was able to rotate and introduce Arnaud Kalimuendo and Jair Cunha for the second half.
McAtee had the best opportunity to add a third when Luiz delicately laid him off. However, the England Under-21s captain blazed his effort over the bar ten minutes after the restart.
He almost rectified his error with an audacious and deft flick for Kalimuendo, who would have scored his first Forest goal before it was chalked off for offside.
You could still not write the hosts off from the contest and Swansea were deservedly rewarded for their persistence to lock horns with Forest.
Eom's in-swinging delivery from a corner found Cameron Burgess who halved the deficit with a powerful header, and it was all about preventing the momentum from swaying away The Reds thereon.
Postecoglou turned to his star-studded bench before conceding, and substitute Callum Hudson-Odoi had his effort tipped round the post in his first involvement, bidding to regain a healthier lead.
Yates, Kalimuendo, and Hudson-Odoi all had near misses and it seemed they could pay the price when Sheehan opted to bring on talisman Zan Vipotnik to attempt to fully turn the tide.
Kalimuendo tumbled to the turf in stoppage time in search of a penalty, which never came, and Forest were made to rue their flurry of chances in stoppage time.
Liam Cullen flashed a teasing ball into the front post, and Vipotink wandered in front of his defender and executed a sumptuous flick to deny Forest from advancing straight through, but all was not said and done.
Ethan Galbraith thundered a fierce strike against the woodwork, and Burgess was there for the rebound to complete a remarkable late turnaround.
Forest were made to pay for their missed chances, and this culminates in a second successive loss in the embryonic stages of Postecoglou's tenure.