Sunderland spent more than a year tracking Virgil van Dijk when the centre-half was catching eyes north of the border.
In one of those quirks of fate, Van Dijk's stoppage time leveller for 10-man Southampton could have inflicted a huge blow on Sunderland's survival hopes after Sam Allardyce's men had been heading for three vital points.
Substitute Jermain Defoe's 85th minute goal looked to have secured all three points at St Mary's, moments after Southampton had been reduced to 10 men.
That goal had Sunderland heading for a win which would have sent them three points clear of relegation rivals Newcastle and Norwich, who both suffered defeats.
But in the 93rd minute, van Dijk finally found a way past Sunderland keeper Vito Mannone, who had produced a string of superb saves to keep the hosts at bay.
There was little quality of note from either side in the opening stages, with both struggling to string a significant sequence of passes together.
The game's first meaningful effort arrived in the 27th minute and it took a superb save from Mannone to prevent Southampton breaking the deadlock.
Sadio Mane floated a cross into the path of Graziano Pelle, who cushioned a beautifully deft header into the path of the onrushing Dusan Tadic.
The winger blasted a half-volley into the turf which bounced up awkwardly for Mannone, but the Italian did excellently to tip it over the bar.
Five minutes later, Sunderland were denied by the woodwork after Khazri drifted in an inswinging free-kick from the left which completely deceived Fraser Forster, but the keeper was able to readjust and get the feintest of touches to divert the ball onto the far post.
Sunderland went close again in the 38th minute when Jan Kirchhoff picked out the run of Patrick van Aanholt down the left.
Van Aanholt's cross was met by Jack Rodwell, who managed to readjust on the volley, but sent his effort wide of the post.
Moments later, Mannone was called upon to make superb save at the other end after Bertrand got in behind the Sunderland defence and delivered a low left wing cross.
Midfielder Oriol Romeu met it with a stabbed effort, but Mannone was able to block from point-blank range.
Fabio Borini was a whisker away from netting a second screamer in as many games early in the second half when the ball came out to him on the edge of the Southampton area.
Borini chested it down and hit a dipping volley in the same movement which landed agonisingly on the roof of the net.
The ex-Liverpool man went close again in the 58th minute when he was fed by Yann M'Vila down the right-hand side of the area and hit a powerful angled drive across goal which was well blocked by Forster.
With 10 minutes to go, Southampton were reduced to 10 men after Jose Fonte was given his marching orders with a straight red card.
Kirchhoff played a wonderful first-time pass towards Borini, whose first touch took him in behind the Southampton defence, where he was hauled down on the edge of the area by Fonte.
Referee Neil Swarbrick had no hesitation and sent the centre-half off.
From the resulting free-kick, van Aanholt curled in a left-footed effort which was smartly pushed away high to his right by Forster.
Moments later, substitute Seb Larsson spooned a left-foot shot over the bar from 15 yards after receiving Borini's pull-back.
But with five minutes to go, the goal did arrive for Sunderland after Lamine Kone fed the ball to Defoe eight yards out.
The 33-year-old tucked it beyond Forster into the far corner in front of the travelling supporters, who went absolutely berserk.
Southampton weren't finished though, with Cuco Martina drawing a smart save out of Mannone with a shot from the right-hand side of the area.
The equaliser eventually arrived in the third minute of stoppage time when Tadic pulled the ball back from the left-hand by-line and van Dijk connected with a powerful first-time shot which beat Mannone at his near post.
Southampton: Forster, Clasie (Juanmi 65), Fonte, S Davies (Ward-Prowse 65), Mane (Yoshida 83), Tadic, Romeu, Martina, Van Dijk, Pelle, Bertrand. Subs not used: Stekelenburg, Cedric, Reed, Olomola. Booked: Bertrand (8), van Dijk (31). Sent off: Fonte (80)
Sunderland: Mannone, Yedlin, Kaboul, Kone, van Aanholt, Kirchhoff (O'Shea 90), M'Vila, Rodwell, Khazri (Larsson 76), Borini, N'Doye (Defoe 59). Subs not used: Pickford, Jones, Toivonen, Mandron. Booked: Khazri (11)