Going to work or school tomorrow morning could be a dispiriting experience for Manchester City followers after a dismal performance here left them at the mercy of their neighbours. Manchester United can move 12 points clear at the top of the table if they beat Everton this afternoon, which would end City's already dwindling hopes of retaining their title.
Southampton, unluckily beaten at Old Trafford the week before last after yet again scoring first, finally kicked a distressing habit of giving away a lead to achieve by far their best win of the season.
Encouraged by their new Argentinian manager Mauricio Pochettino to harass the opposition high up the pitch and win the ball back as quickly as possible, they did just that to score twice in the first 21 minutes and, even after conceding to Edin Dzeko, benefited from a catastrophic own goal by Gareth Barry right at the start of the second half.
A despairing City were deserving of their fate which, after successive draws with Queens Park Rangers and Liverpool, is to remain nine points behind United, whatever happens today.
They suffered badly from the continued absence in defence of Vincent Kompany, with Javi Garcia, in a rare appearance there, proving no substitute. He and Joleon Lescott were uncomfortable throughout the time they shared defensive duties against two aggressive old-school strikers in Rickie Lambert, seeking his 100th goal for the club, and Jay Rodriguez, while Gael Clichy at left-back was poor throughout.
Barry, left out of the England squad all season, also had a game to forget, his negligence negating the positive effect of Yaya Touré's return alongside him after Africa Cup of Nations duty with Ivory Coast, and Samir Nasri was just one of the obvious candidates to be replaced when Roberto Mancini decided on changes at 3-1 down. Carlos Tevez, who could have expected a recall, was missing for what were described as personal reasons.
So the Hampshire club moved six points clear of the relegation places by beating a team in the top half of the table for the first time. That should do their self-belief as much good as not adding to a record of having lost no fewer than 24 points by losing leads in previous games.
Moreover, Pochettino now has a first victory to his name, after taking over in unfortunate circumstances from the popular Nigel Adkins, whose tough start to the season included two narrow 3-2 defeats by the Manchester clubs.
On the opening day back in August, the newly promoted Saints led 2-1 after 70 minutes before goals by Dzeko and Nasri. The lead came earlier this time, only six minutes from the start. Jason Puncheon robbed Barry, and as Lambert made a clever run to the right the winger fed Rodriguez in the resulting space on the left. Joe Hart did well to save, but Puncheon followed up to score.
With less than a quarter of the game played it was 2-0l. This time Lambert played a one-two with Puncheon and cut too easily inside Clichy on to his left foot. Hart carelessly spilled the resulting shot and Steven Davis pounced on the rebound, just ahead of Pablo Zabaleta's lunge.
It was more than half an hour before Zabaleta, one of City's few impressive performers, curled a shot wide of the far post. The way his team scored seven minutes before half-time nonetheless reflected well on them and badly on Southampton. The home side had won a corner from Nathaniel Clyne's deflected shot but took an over-ambitious one designed for a player on the far side to volley at goal. Instead, David Silva won possession and set up a perfect counter-attack in which he featured twice, feeding Dzeko then the overlapping Zabaleta, whose low cross was turned in by Dzeko.
Southampton still felt with some justification that they were hard done by just before half-time when Yaya Touré barged over Rodriguez in the penalty area. It hardly mattered, given the way they resumed after the interval.
First Puncheon drove over the bar from Lambert's pass and then the latter, given all the space he wanted to take Davis's pass on the left, misplaced a cross that Barry, with time to play a straightforward back-pass, side-footed past Hart into an unguarded corner of the net.
Mancini, having one of those days of anguished Italianate gesticulation in the technical area, could have chosen any number of players to bring off when he sent on James Milner; he chose the ineffectual Nasri. Milner was lively, although the next change, putting on Aleksander Kolarov for Joleon Lescott, smacked more of desperation to prevent a League championship disappearing over the horizon. That is where it will be if United win at home today.
A delighted Pocchetino said: "It's been an incredible day for all the people who came to Southampton today." In a different sense it was an unbelievable one for Mancini.
Southampton (4-4-2): Boruc; Clyne, Yoshida, Hooiveld, Fox (Richardson, 71); Puncheon (Lallana, 61), Davis (Ward-Prowse, 80), Schneiderlin, Cork; Lambert, Rodriguez.
Man City (4-4-2): Hart; Zabaleta, Garcia, Lescott (Kolarov, 66), Clichy; Nasri (Milner, 55), Yaya Toure, Barry, Silva (Maicon); Aguero, Dzeko.
Referee: Martin Atkinson
Man of the match: Puncheon (Southampton)
Match rating: 7/10