The Saints scored two goals in the space of three minutes late on to complete a magnificent turnaround at the Tottenham Hotspur Stadium.
Tottenham had already had a goal ruled out and missed a sitter by the time they took a 17th-minute lead.
A slick move saw Pierre-Emile Hojbjerg get in down the right hand side and his low cross was turned into his own net by Jan Bednarek.
The visitors were level five minutes later as Chelsea loanee Armando Broja fired home from close range after Ben Davies had missed his kick in the six-yard box.
The Albania international proceeded to celebrate in front of the Spurs fans.
Southampton looked the more likely to go in front, but the teams remained locked at 1-1 at the break.
It was Heung-min Son who put Spurs ahead on his Premier League return, but then came the collapse at the back.
Mohamed Elyounoussi levelled in the 79th minute, before Che Adams struck Southampton's third just four minutes later.
But the drama didn't end there as Steven Bergwijn thought he had scored an equaliser to rescue his side again.
He put the ball into the back of the next, only for VAR to intervene and rule Bergwijn was offside.
Hasenhuttl's side took all three points in a thrilling clash at the Tottenham Hotspur Stadium.
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