Mohammed Salisu's shambolic own-goal epitomised the Saints' defensive efforts as they were simply overrun and unable to hold onto their early advantage.
The match began brightly for Spurs as they were on the front foot in the first ten minutes, but quickly found themselves behind against the run of play.
James Ward-Prowse temporarily silenced the Tottenham Hotspur Stadium with the opener after 12 minutes.
Moussa Djenepo did well on the left to get away from Emerson Royal and chipped to the edge of the area where Southampton captain Ward-Prowse volleyed home to put the visitors ahead.
"What a volley from James Ward Prowse," said talkSPORT's Perry Groves on commentary, "Really good play from Southampton.
"Emerson Royal should do better to stop the cross."
Nine minutes later though, Spurs were level.
Dejan Kulusevski checked back onto his left foot and produced a wonderful cross from the right.
The ball found Ryan Sessegnon who beat Kyle Walker-Peters in the air to head home from close range and make it 1-1.
By the 31st minute, Tottenham had turned the game on its head.
Southampton cleared Heung-min Son's initial corner, however the ball came back to the South Korean and his cross was smartly headed in by a stooping Eric Dier to make it 2-1.
Groves reacted: "Well the defending from Southampton has been dire in the wide areas and Dier makes them pay for that.
"That's such a clever header from Dier. On the balance of play, Spurs deserve the 2-1 lead."
At the start of the second half, Sessegnon briefly thought he'd bagged a brace, only for the offside flag to deny him.
Tottenham did make it 3-1 in the 61st minute thanks to a helping hand from Mohammed Salisu.
Royal led the breakaway and exchanged passes with Son before his scuffed cross was shambolically put into his own net by Salisu.
"It's really poor defending from Salisu," Groves said.
"Royal was just onside, he's flashed it across with his left foot and Salisu can't get his feet sorted out.
"He's completely off-balance, it looks like the ball's taken him by surprise and he's actually had a long time to see the ball coming to him.
"That's really poor defending from Salisu."
Two minutes later, Spurs increased their advantage to three goals.
Royal was released again and cut back for Kulusevski, who opened up his body and curled into the bottom corner to cap a wonderful individual display.
Groves reacted: "Southampton have completely lost their shape, lost their belief.
"Kulusevski has no Southampton player with him, gets the ball onto his left foot, and how composed he is, opens his body up, passes the ball past Bazunu.
"Shocking defending from Southampton, but clinical finishing from Tottenham Hotspur."
Fortunately for the visitors, there were no further home goals and the game ended 4-1.
"I've got to mention Kulusevski," Groves pointed out at full-time, "I thought he was absolutely outstanding.
"He was dangerous. One goal, one assist."