Swansea's Nathan Dyer
A threat throughout, his strike silenced the jeers that erupted every time the elusive 21-year-old from Pompey's bitter south coast rivals touched the ball.
The tireless Jason Scotland, who set up Dyer, inflicted further misery on Tony Adams' side when he slotted home a penalty in first-half injury time.
It was with some justification that Nadir Belhadj protested furiously when referee Andre Marriner pointed to the spot as his challenge on Jordi Gomez looked outside the box.
Egos dented, Pompey subjected the Coca-Cola Championship side to a fearful battering after the interval.
As in the first half, the reflexes of Swansea's heroic keeper Dimitrios Konstantopoulos and stout defending denied them on a host of occasions.
Peter Crouch cut a frustrated figure as Konstantopoulos repelled everything the England marksman could throw at him, while during nerve-jangling final spell, Pompey were kept out time and again.
The warning signs for the Barclays Premier League club were there with just a few minutes on the clock, Angel Rangel shooting narrowly wide of the left post.
Dyer set off on a marauding run down the left flank that saw him find acres of space and tee-up Jordi Gomez, who directed his effort straight at David James.
Pompey debutant Jermaine Pennant was forced to try his luck from long range, evidence of a bright opening spell from Swansea who looked marginally the more threatening side.
Jason Scotland puts Swansea 2-0 up at Portsmouth
Keeping the ball on the deck at all times, Swansea were having little problem picking Pompey apart and they fully deserved to take the lead.
And the worst fears of home supporters were realised when it was Dyer who smashed the visitors ahead.
Collecting Scotland's cute floated pass on the edge of the area, Dyer chested the ball down and drilled it past James into the bottom right corner.
The busy Scotland - an imposing figure at the tip of Swansea's attack - deserved his assist, but it was not all one-way traffic as Pompey probed for an equaliser.
Crouch was denied by the lightening reactions of Konstantopoulos and then centre back Alan Tate produced a timely block on an attempt by Pennant.
The pendulum continued to swing as half-time approached, Swansea's defence coming under growing pressure.
But in their eagerness to pour forward, Pompey were caught on the counter in first-half injury time and paid the price as the Championship side extended their lead.
Gomez dashed into the box and tumbled beneath the tackle Belhadj, who protested his innocence as Marriner pointed to the spot before Scotland converted.
New Portsmouth signing Jermaine Pennant avoids a tackle
Pompey boss Tony Adams brought on Kanu and Hermann Hreidarsson for the second half in an effort to change his side's fortunes.
Crouch tumbled beneath a challenge but Marriner was not interested in the shouts for a penalty and moments later Konstantopoulos had to dive to keep out a header from the towering forward.
Swansea were weathering a fearsome early assault from Pompey and were grateful when Dyer's break and shot at James helped relieve the pressure.
A fingertip save from James kept out Gomez and there were dramatics at the other end as Konstantopoulos produced a sublime save before causing panic by dithering in goal.
Crouch struck the woodwork as Swansea continued to ride their luck amid a series of frantic goalmouth scrambles.
With six minutes to go Pompey, sensing their time was up, took their foot of the gas to leave their cup defence in ruins.