Martin O'Neill's credentials as a manager have rarely been questioned as much as they have this season after an atrocious run that left Sunderland perched a point above the bottom three, so it was not hard to understand the relief after a battling victory over their fellow strugglers Southampton, who simply cannot afford defeats of this nature if they wish to stay up. Nigel Adkins's side remain out of the relegation zone on goal difference but that may not be the case for long if there are many more performances as lethargic as this.

Wigan's defeat by Arsenal earlier in the day had given Southampton the perfect opportunity to put some breathing space between themselves and the bottom three for the first time this season and there was plenty of optimism in the air after Southampton's recent upturn in form. Ninety minutes later and they were contemplating Sunderland – who moved up to 15th, four points clear of Southampton and Wigan – leaving with the points thanks to Steven Fletcher's goal just before half-time.

"The game was really tight," O'Neill said. "Southampton are in good form. It was important to get the first goal. It's been a tough old season right from the start. We've got a way to go yet."

This was a victory that had more to do with perspiration than inspiration. For long periods, Sunderland were pinned back by Southampton's high-octane style yet their defence, marshalled by the superb Carlos Cuéllar, stood firm and perhaps the most disappointing aspect of the afternoon for Adkins was that too many of his players did not leave everything out on the pitch. By contrast, O'Neill, as hyperactive as ever on the touchline, could be seen dragging every last ounce of effort out of his side. Gastón Ramírez, Southampton's most expensive signing, would do well to take note.

Admittedly Ramírez was often the catalyst for Southampton's best moments in the first half and he had to be considering the absence of their influential captain, Adam Lallana, with a knee injury. The Uruguayan brought the best out of Simon Mignolet with a swerving effort and also created an opening for Emmanuel Mayuka which the Zambian, making his first league start, wasted. Rickie Lambert also went close with a fizzing free-kick that deflected wide off Craig Gardner.

Sunderland's threat was intermittent and it seemed their best moment of the half would remain Stéphane Sessègnon's brilliant turn and shot from 25 yards out that Kelvin Davis somehow tipped wide in the first minute. Yet just before half-time, they struck. James McClean released Adam Johnson in behind the dozing Nathaniel Clyne and, although Sessègnon miscued comically, the ball fell to Fletcher who converted from close range for his eighth goal of the season.

"He's been fantastic for us," O'Neill said. "I'm glad Sessègnon 'passed' to Fletcher. All the goals he's scored have been big goals."

Under O'Neill, Sunderland have turned losing leads into an art form. But after Adkins introduced Steve De Ridder early in the second half, Southampton poured forward without really suggesting they could find a way through the Sunderland defence.

"The normal buzz we have didn't seem to be there," Adkins said. "It was uncharacteristically flat. One or two players could have done better stopping the goal."

Ultimately the difference between the two sides was summed up by John O'Shea throwing himself heroically in front of Guly Do Prado's goalbound volley in stoppage time, thereby sealing a victory that could ignite Sunderland's stuttering season and spell doom for Southampton's.