A dream start was long forgotten as Saints were demolished by Liverpool and dumped out of the Capital One Cup.
Sadio Mané sparked wild celebrations by giving Southampton the lead after only 39 seconds, but such jubilation was a distant memory by the time the final whistle blew at St Mary's.
Two goals in the space of five minutes from Daniel Sturridge just before the half-hour mark put the Reds ahead, before Divock Origi added another on the stroke of the interval.
Saints showed signs of life after the break but their revival proved temporary, as two more goals in quick succession – from Origi and substitute Jordon Ibe – put the visitors in cruise control.
And Belgian Origi rounded off his hat trick with four minutes remaining to complete a dismal night for Ronald Koeman's men and dispel any dreams Saints had of winning this particular competition.
Koeman had named a strong starting XI for the tie, drafting in the likes of Cédric, Dušan Tadić and Jordy Clasie after they were dropped for Saturday's defeat at Manchester City.
Liverpool made changes of their own, giving chances to backup goalkeeper Adam Bogdan and young defender Connor Randall for what was their third match in the space of a week.
If there was any weariness in the Reds' legs, it showed early on as Saints took the lead inside a minute. Tadić showed good feet before laying the ball back to Ryan Bertrand, whose hanging cross was headed in by Mané at the back post.
With St Mary’s bristling, Saints looked to make the most of their early ascendancy. Tadić bamboozled once more, cutting onto his right to pick out the run of Victor Wanyama only for the Kenyan’s header to be beaten away by Liverpool goalkeeper Bogdan.
Tadić continued to dazzle on the left, drawing the attention of the Reds’ young full-back, Randall, who was shown an early yellow for slicing down the Serbian.
Then – still with less than ten minutes on the clock – Mané combined with Graziano Pellè to tee up Steven Davis, who curled a shot over.
Saints’ dominance ended there, though. The visitors gradually found their feet and, just passed the midway point of the half, they duly levelled.
Joe Allen’s perfectly-lofted pass set Sturridge clear of Steven Caulker and, even after a poor first touch, the forward was able to shift the ball wide of Saints’ recovering centre-half before drilling past Maarten Stekelenburg with 25 minutes gone.
The game was turned on its head soon afterwards. Another superb pass, this time from Emre Can, released Sturridge, who wasted no time in slotting beyond Stekelenburg.
Saints were denied an almost-immediate reprieve when, after Tadić’s prodded pass came back off the already-cautioned Randall’s arm, referee Robert Madley opted not to award a penalty.
With Koeman’s men growing more and more frustrated, Liverpool added another goal at the start of first-half stoppage time.
Ex-Saint Adam Lallana’s corner was headed out only as far as Alberto Moreno, whose driven effort took the slightest of touches off the boot of an offside-looking Origi to extend the visitors’ advantage.
Southampton started the second period well, with Pellè testing Bogdan and Mané having a shot blocked either side of Lallana sending a speculative effort over Stekelenburg’s goal.
Reds boss Jürgen Klopp withdrew Sturridge just ahead of the hour mark, shortly before Koeman rolled the dice himself by introducing Shane Long and James Ward-Prowse for Clasie and Cédric.
Long was involved straight away, rising to meet a delivery from Tadić – who had, again, toyed with Randall on the left – only to flick the ball wide of Bogdan's goal.
The knockout blow arrived soon afterwards, though. Jordon Ibe, on for Sturridge, played a superb through-ball to release Origi to smash beyond Steklenburg for the Reds' fourth.
Goal five wasn't far away. With Saints' defenders seemingly abandoning their positional sense, Moreno crossed from the left for Ibe to take the ball on his chest and rifle home with still more than a quarter-hour remaining.
The remainder of the match was played out at a relative canter, with Origi's 86th-minute completion of his hat trick – a close-range header from a left-wing cross – the only action worthy of remarking upon.
Come full time, the sight of a half-empty St Mary's signified a dismal night for the club – and one made all the worse by just how well it started.
Southampton (4-3-2-1): Maarten Stekelenburg; Cédric (James Ward-Prowse 63), Steven Caulker, Virgil van Dijk, Ryan Bertrand; Victor Wanyama, Jordy Clasie (Shane Long 63), Steven Davis (c) (Oriol Romeu 77); Sadio Mané, Dušan Tadić; Graziano Pellè. Unused substitutes: Paulo Gazzaniga (GK), Maya Yoshida, Cuco Martina, Juanmi.
Goal: Mané (1)
Liverpool (4-3-1-2): Adam Bogdan; Connor Randall, Martin Skrtel, Dejan Lovren, Alberto Moreno (Brad Smith 77); Lucas Leiva (c), Joe Allen (Jordan Henderson 74), Emre Can; Adam Lallana; Divock Origi, Daniel Sturridge (Jordon Ibe 59). Unused substitutes: Simon Mignolet (GK), James Milner, Christian Benteke, Roberto Firmino.
Goals: Sturridge (25 & 29), Origi (45, 69 & 86), Ibe (73)
Booking: Randall (8), Can (65)
Referee: Robert Madley
Attendance: 31,592 (3,151 away fans)