Heung-Min Son's 87th-minute penalty sent Tottenham into the FA Cup fifth round as they beat Southampton 3-2 in their fourth-round replay.
An early own goal from Jack Stephens (12) on his 100th Southampton appearance handed Tottenham the advantage, but Shane Long (34) equalised before the break.
Danny Ings deservedly fired Southampton ahead in the 72nd minute, but not long after, Lucas Moura (78) levelled things up.
As it looked like the game was heading to extra-time, Son converted from the spot after being bought down by Angus Gunn in the area to send Tottenham into the fifth round, setting up a home tie against Norwich.
Southampton will be wondering what happened as they had been by far the better side, yet collapsed after going in front, and they are now left to focus on pushing for a Europa League place in the Premier League.
Having seen Ings and Nathan Redmond shoot straight at Hugo Lloris in the opening 10 minutes, Spurs were the recipients of a large slice of luck at the other end as they took the lead.
Harry Winks' long ball allowed Ryan Sessegnon to burst into the box, but Stephens brilliantly denied him a shot with a sliding tackle.
However, he was not rewarded as the ball fell to Tanguy Ndombele on the edge of the area and the Frenchman's shot, which was going wide, cannoned off Stephens on the floor and into the bottom corner.
It was cruel on Southampton but they were not deterred and continued to create regular chances. One of them fell to Ings and he would have expected to score as James Ward-Prowse's long ball sent him in on goal, but the in-form striker hit the crossbar.
The deserved equaliser came in the 34th minute as Saints sliced their visitors open again. Redmond was given too much space to get a shot away, which Lloris could only parry into the path of Long, who made no mistake with the rebound.
The second half started in a similar fashion and only a bad miss by Sofiane Boufal stopped the visitors taking the lead as he scuffed his shot at the far post and could not even hit the target.
Spurs had been virtually non-existent as an attacking threat, yet they created a brilliant chance to go in front as Son's cross was begging to be headed home by Moura, but five yards out in the middle of the goal he somehow put it wide.
That was a fleeting moment for Spurs as Southampton continued to look the most likely, with Jannik Vestergaard forcing Lloris into a flying save with a header.
And they finally went ahead 18 minutes from time with a breakaway goal of stunning quality. Redmond, picking the ball up on the edge of his own area, beat four men before sliding in Ings, who cut inside and clinically found the bottom corner.
It looked desperate for Spurs, but to their credit they mounted a fine response, inspired by Dele Alli. The substitute first played a one-two with Moura, before the Brazilian created some pace and fired into the bottom corner.
They then won it in the 88th minute as Son scored his fourth goal in four games. A fast break saw Alli play the South Korean in, he rounded goalkeeper Angus Gunn, but was brought down and referee David Coote pointed to the spot. Son was the man who converted as Spurs edged through to the next round.
What the managers said
Tottenham manager Jose Mourinho: "The best team lost. The best team on the pitch lost. If you look at it in a pragmatic way, forgetting that they had their best players, they had their full team, they had one more day to rest, we were really in trouble.
"We were in real, real trouble to build a team, lots of fatigue after the match against Manchester City... if you forget these important items and you analyse in a simple pragmatic way, the best team lost.
"They were the best team, but I cannot forget all the other circumstances, and my players, they were phenomenal. The way they react was fantastic. They showed incredible heart and had a great connection with the crowd."
Saints boss Ralph Hasenhuttl: "It's nice that he praises us, but it doesn't help me. Not every game of football is about being the better team, it is about being the winning team. And we're not.
"It is easy for him to praise us. They were more clinical because in the last 15 minutes they turned the table in the way that is part of a good team. We couldn't defend how we wanted to because this is very disappointing for me. Being strong for 75 minutes is not enough."
Opta stats
- Spurs have progressed from each of their last 11 replays in the FA Cup, with six of those being achieved over fellow top-flight opposition.
- Spurs have only kept three clean sheets in 19 games under Jose Mourinho - the only Premier League sides to have managed fewer across all competitions in this period are Bournemouth (2) and Brighton (1).
- Since the start of the 2016-17 season, Spurs' Son Heung-Min has scored more goals than any other player in the FA Cup - netting 11 goals in 17 appearances.
- Danny Ings has now scored 17 goals in all competitions in 2019-20; the most by a Southampton player in a single season since 2013-14 (also 17 - Jay Rodriguez).
What's next?
Tottenham embark on their mid-season winter break before returning to Premier League action at Aston Villa on Sunday February 16, live on Sky Sports, at 2pm.
Southampton also enjoy a pause before resuming their league campaign at home to Burnley on Saturday, February 15; Kick-off at 12.30pm.