Steven Fletcher's clinical touch will be key in deciding whether Sunderland stay away from the threat of relegation. It was therefore crucial when he passed a late fitness test on an ankle injury ahead of this important match.
This season, the striker's form has indicated that he does not need many attempts to score and that was reinforced here. It was not clear whether Stephane Sessegnon's touch on for Fletcher's goal was an intentional moment of exceptional vision and class, or just a scuffed shot. Sessegnon's facial expression suggested the latter but either way the ball fell to Fletcher, 10 yards out, and the end result was predictable as he tucked it in.
After snatching that lead just before half-time Sunderland dropped deeper and managed to complete a hard-fought and deserved win. This was the first time in four months that Southampton had failed to score at home and, apart from a late flurry of chances, their usual attacking exuberance was stifled.
This away victory for Sunderland followed their triumph at Fulham last month. "It's been a tough old season and I feel that win will give us a bit of confidence," said Sunderland manager Martin O'Neill. "All the goals Fletcher has scored for us have been big goals."
Looking ahead to a daunting holiday-period fixture list - with Manchester City next on Boxing Day - O'Neill urged the fans to keep faith. "If we can bring that support to the game, the atmosphere will be akin to New Year's Day when we beat Manchester City," he said.
Sunderland have developed a reputation as slow starters, but they took only 22 seconds to open up Southampton. Danny Rose threaded the ball through for Sessegnon, who turned sharply and powered a 30-yard strike towards goal before Kelvin Davis tipped over.
Southampton threatened when Rickie Lambert curled a 20-yard free-kick towards goal, but Craig Gardner's touch diverted it off target and Gaston Ramirez - looking to provide spark in the absence of injured Adam Lallana - tried his luck with a couple of ambitious efforts. The game had quickly opened up, with both teams looking lively.
There was a slight drop in urgency just before the break but, just when proceedings were becoming scrappy, Sunderland broke through. Adam Johnson advanced down the left wing, up to the byline, then slipped the ball into the area where Sessegnon provided his touch for Fletcher to finish.
Early in the second half, Lambert had a glancing header from Steve de Ridder's corner cleared by Jack Colback, but Sunderland began to close ranks. Despite that, they nearly humiliated Southampton goalkeeper Davis as he made a mess of gathering Gardner's long-range free-kick. He dropped the ball, which looked set to squirm through his legs, but recovered.
Southampton spurned late openings, Ramirez having a shot charged down by Fraizer Campbell, then Guly Do Prado seeing a strike deflected wide.
"It appeared we were uncharacteristically in a flat game," said Southampton manager Nigel Adkins. "The buzz we normally have wasn't there."
Southampton (4-2-3-1): K Davis; Clyne, Fonte, Yoshida, Shaw; Schneiderlin (S Davis 59), Cork; Puncheon (Do Prado 80), Ramirez, Mayuka (De Ridder 54); Lambert. Subs: Boruc (g), Hooiveld, Rodriguez, Richardson.
Sunderland (4-4-2): Mignolet; Gardner, Cuellar, O'Shea, Rose; Johnson (Campbell 75), Larsson (Bardsley 70), Colback, McClean; Sessegnon (Vaughan 89), Fletcher. Subs: Westwood (g), Wickham, McFadden, Bramble. Booked: Larsson, Johnson, Bardsley.
Referee: H Webb (Yorkshire).