Carlos Alcaraz’s first-half winner lifted Southampton off the bottom of the Premier League table and within a point of safety on a tense evening at St Mary’s.
Saints could even afford the luxury of a rare missed penalty from James Ward-Prowse in a first half the hosts shaded.
Alcaraz’s second goal for the club, which survived a marginal VAR decision for offside against Ché Adams before the striker played him in, arrived three minutes after the spot-kick and ten before the interval.
Leicester will be wondering how Kelechi Iheanacho did not score – or even test Gavin Bazunu – from one of his three clear sights of goal, but the Nigerian was wasteful, while Harry Souttar rattled the crossbar in the last of five agonising added minutes, as Saints held on for a second 1-0 win in three Premier League outings.
Rubén Sellés made four changes to his last league line-up, including a first start in four months for Theo Walcott and a welcome return from injury for Adams, who led the line flanked by Walcott and Kamaldeen Sulemana.
Elsewhere, Kyle Walker-Peters and Alcaraz returned, as Romain Perraud, Stuart Armstrong, Mohamed Elyounoussi and Paul Onuachu dropped to the bench.
Sellés would have been encouraged by Saints’ start, particularly when a surging run down the right wing carried Ainsley Maitland-Niles all the way into the penalty area where he picked out Theo Walcott, but Walcott’s goal-bound shot from ten yards struck teammate Kamaldeen, offering Leicester a lucky escape.
The visitors’ response was a cute pass from James Maddison to set Ricardo Pereira free inside the box, but the right-back appeared to be caught in two minds, failing to shoot on target or pick out Iheanacho, who was handily placed on the six-yard line.
It was from that same position that the Nigerian should have put the Foxes in front in the 20th minute. Perhaps he saw Maddison’s free-kick late, as it fizzed over the head of Wout Faes at a rate of knots, but Iheanacho somehow contrived to divert a golden chance wide.
Leicester had edged the early exchanges, but Saints were about to pass up the best opportunity to date when Ward-Prowse was uncharacteristically thwarted from the penalty spot.
It was Walcott’s one-two with Adams that set up the chance, before the winger’s cross from the byline struck the raised arm of left-back Timothy Castagne.
It was harsh on the Belgian, who was merely trying to block the cross, but his arm was up and the decision could not be disputed, leaving Ward-Prowse with the ball in his hand.
But it was the away fans celebrating when Ward read the intentions of the Saints skipper, diving to his right to repel the spot-kick, which was well struck but halfway up the goal, just where Ward would want it, and not close enough to the corner.
It may not have been the outcome they wanted, but it still stirred something inside the Saints faithful, who roared their team forward again.
The players responded, as Alcaraz stooped to head a Ward-Prowse cross into Ward’s arms, before Kamaldeen dragged a left-footed shot wide as Saints upped the ante.
Their reward arrived ten minutes before the interval. Adams dropped short to collect a low pass and quickly turned to release Alcaraz in one movement, who had run beyond him and was now sprinting towards goal with only the keeper to beat.
As Ward advanced and defenders retreated, the Argentinian kept his cool to finish with a low shot across the Welsh stopper from the edge of the penalty area.
Such was the joy with which he celebrated, before the subsequent VAR check ruled in Saints’ favour, Alcaraz injured his knee – struggling through to half time only to be withdrawn soon after the break.
There was no doubt who was on top at this point, as Kamaldeen tested Ward once more, while Leicester’s two most creative players, Harvey Barnes and Maddison, both went into the book as the visitors’ frustrations grew.
The shift in momentum prompted a half-time substitution from Brendan Rodgers, introducing Dennis Praet for Barnes, whose cross immediately after the restart was begging to be headed home by Iheanacho, but again the striker misfired from close range.
By the hour mark Saints were getting pushed back, prompting Sellés to bring on some fresh legs up front in Sékou Mara and Adam Armstrong, as Adams and Walcott departed to deserved applause.
But the chances kept coming Leicester’s way – specifically the way of Iheanacho, who was guilty of another bad miss midway through the second half.
This one was all Saints’ own making, as Bazunu scuffed a clearance straight to the Leicester man just outside the box, who had time to compose himself and pick his spot, but his shot was too high, grazing the top of the net on its way over.
When Rodgers turned to Jamie Vardy soon after, it was not to replace Iheanacho but to partner him, as the visitors threw caution to the wind, sensing their opportunity to strike.
Saints were having to pick their moments to counter-attack, but in Kamaldeen they have a roadrunner ready to burst into life.
The winger set off down the left, leaving the Leicester defence in his wake before squaring the ball across goal for Armstrong, but the substitute lifted the chance high over the bar, and would have been relieved to see the offside flag spare his blushes.
Meanwhile, Iheanacho resumed his duel with the target, but still could not force Bazunu to make a save, as Leicester’s pressure mounted.
The scheming Maddison would not go away either, as the Leicester captain crossed for Faes to head wide before Saints survived the biggest scare of all, deep into stoppage time, when Bazunu misjudged another Maddison delivery and Souttar’s header thundered against the crossbar.