Almost 31,000 fans crammed into St Mary's to watch Alan Pardew's side defeat second-placed Leeds with the scoreline not reflecting Saints' dominance throughout the game.
Results elsewhere mean that Saints remain 16 points off the play-offs with a game in hand but with increased confidence ahead of the visit of sixth-placed Swindon on Tuesday night.
Gary McSheffrey had the first shot of the game after just a minute but from 25 yards out he dragged it wide.
Up the other end Rickie Lambert had his first effort a minute later but leaning backwards to reach Dean Hammond's chipped cross he directed it off target.
Jason Puncheon tried an audacious shot from just inside the area on five minutes, curling it just wide with the outside of his left boot after a forceful run and incisive pass from Adam Lallana.
It was an exciting start to the game with both teams getting the ball down and passing it around at tempo and quality that belied the division they are competing in.
McSheffrey continued the action by firing in a fierce free kick that was going to trouble Kelvin Davis before it was inadvertently blocked by his own player, Luciano Bechhio, on the edge of the six-yard box.
Radhi Jaidi had a chance when he was picked out unmarked by Jon Otsemboor's deep free kick on 11 minutes but put his header over.
Up the other end Jermaine Beckford fired in a shot from 25 yards that whistled just wide.
Hammond extended Kasper Ankergren for the first time on 15 minutes with a low shot drilled straight down the middle that the Leeds keeper could only parry with Patrick Kisnorbo just edging it away from Lallana following in for the rebound.
Saints were forced into their first change moments later when Otsemobor limped off to be replaced at right back by Wayne Thomas.
Hammond was in the thick of the action again on 17 minutes as he fired over from the edge of the area after Puncheon's cut back.
Puncheon came even closer himself on 19 minutes when he cut in from the right and shot from 20 yards left footed. His low effort beat Ankergren but clipped the outside of the post.
But there was no stopping Saints when they took the lead on 25 minutes.
A left wing cross came to Lambert at the far post, he managed to get it down, turn and cut it back to Puncheon.
The winger scuffed his shot but that turned it into something of a cross into the centre of the six-yard box where Dan Harding was all alone to provide a simple finish for 1-0.
Barnard tried to add another on 32 minutes but his half volley fired off the head of Kisnorbo and over.
Saints did think they had doubled their lead a minute later as Jaidi turned home a corner in a collapsing melee of players at the near post but the referee disallowed it for a foul.
Saints then should have got another goal on 35 minutes as another scramble in the Leeds box saw the ball fall to Barnard but from just a few yards out he could only hit it against the diving body of Aidan White back on the line who blocked it.
There was just as good an opening three minutes later as Puncheon's chipped cross from the right picked out Lallana with a free header in the centre of goal six yards out but he put it over.
Saints were furious with referee Peter Walton as he failed to blow up when Barnard looked like getting through on goal before a clash with Richard Naylor then Jonathan Howson was the first man in the book for a foul on Lallana.
When the half-time whistle went Saints led 1-0 having played some fantastic stuff and created some great chances but knowing they should have had more goals.
Leeds made a switch at half-time to try and get themselves back into the game with Leigh Bromby coming on for White.
The second half started unsurprisingly more subdued.
Saints still looked in control for the most part but Leeds were more compact and defending better.
Kisnorbo picked up a booking for a foul on Lambert as the referee came in for more stick while Puncheon followed him not long afterwards.
Leeds made a second change on 63 minutes with Neil Kilkenny coming on for McSheffrey.
Saints were doing a good job in the second half.
The game was not at all open like the first half with neither side getting a sustained spell of pressure and even half chances proving hard to come by.
But with a lead to protect Saints were ensuring Leeds never got a firm foothold in the game and always remained a threat themselves.
Pardew decided to introduce the threat of some pace on the break when Papa Waigo replaced Barnard on 72 minutes.
Leeds responded with their final change five minutes later with Max Gradel on for Becchio. Gradel got booked almost instantly for a challenge on Harding.
With ten minutes remaining and still only one goal in the game Saints must have been bracing themselves for a sustained spell of pressure from Leeds as they desperately hunted for a goal to get something from the game.
But it was Saints who were so lively on the break with Waigo testing Ankergren with a low shot from 20 yards.
When the fourth official held up the board it showed a minimum of five minutes of added time.
In fairness to Saints they had done a great job up until that point of strangling the game and were certainly not holding on.
They may have been slightly worried when Robert Snodgrass took aim from 25 yards but his shot whistled just wide.
That proved to be Leeds' last foray and Saints took a well deserved and impressive three points from the game.