Southampton were harshly dumped out of the Emirates FA Cup with a one-sided scoreline that did not tell the whole story of a quarter-final that hung in the balance for 75 minutes.
Saints fell behind when a defensive error was punished by Raheem Sterling immediately after Adam Armstrong had struck the post at the other end.
That was after 12 minutes, but the hosts regrouped and gave as good as they got, deservedly drawing level in first-half stoppage time when Aymeric Laporte put through his own net.
A soft penalty decision awarded against Mohammed Salisu allowed Kevin De Bruyne to restore City’s lead just after the hour.
Ralph Hasenhüttl immediately brought on Armando Broja and Ché Adams, and the latter was denied by goalkeeper Zack Steffen from a golden chance shortly before Phil Foden thundered in an unstoppable shot 15 minutes from time.
Foden’s fellow substitute Riyad Mahrez made matters worse for Saints with a quick-fire fourth, but this was a competitive cup tie that hinged on key moments.
Hasenhüttl made four changes to his side from the Premier League meeting with Watford in which Saints fell to a third successive defeat.
Jack Stephens and Tino Livramento were restored to the starting line-up, along with strike duo Shane Long and Adam Armstrong, as Hasenhüttl picked a pacey, energetic side to disrupt the league leaders.
The opening five minutes offered plenty of early encouragement, with Long just unable to take the ball in his stride in the penalty area before Kyle Walker-Peters raced to the byline to deliver a cross.
City seemed slightly taken aback by Saints’ fast start, but still registered the first shot on target when Sterling’s 20-yard curler was comfortably dealt with by Fraser Forster.
The England winger had a much better chance inside the first 10 minutes, fed by Ilkay Gündogan in the penalty area, but Salisu got back quickly enough to hurry the shot, which Sterling slashed wide on his left foot.
Saints were going toe to toe, eager to score first. Armstrong was inches from providing it when slipped through by Oriol Romeu, but he scuffed his shot across the advancing Steffen.
The keeper was beaten as St Mary’s held its breath, but the ball cruelly struck the inside of the far post and bounced back into play.
Within a minute, City were in front as Sterling capitalised on a defensive error to convert his third chance of the day.
When Jack Stephens was unable to clear his lines in the box, Gabriel Jesus was in the right place to pick up the pieces and lay the ball square for Sterling to tuck it in from 12 yards.
It was not a good goal to concede, if there is such a thing, but Saints were not about to go quietly.
More bright build-up play resulted in another chance for Armstrong midway through the half.
This time he had to stretch to control a pass from Long before the ball sat up kindly to be hit, but the striker got his technique wrong, opting for a side-footed half-volley that skewed off target when perhaps drilling the ball with his laces was the better choice – easy to say in hindsight.
At the other end, Saints survived a major scare as City threatened to pull away in the 27th minute.
Jack Grealish spread the ball wide to João Cancelo, who fizzed in a cross met by Gundogan at the near post, who must have thought he’d scored only to slam the ball against the upright from close range.
One lapse aside, Saints had played well, and there was plenty of encouragement for the hosts in the closing stages of the first half.
Livramento delivered a couple of teasing crosses towards Long; the first of which saw the Irishman eased out of the way by Kyle Walker, prompting optimistic cries for a penalty, before Long did get his head on another delivery, but was back-pedalling and could not direct it on target.
The pressure eventually told with just seconds remaining. Stuart Armstrong played the ball in behind the defence for Elyounoussi, who kept the ball, drew Steffen from his line and squared it across goal.
Saints’ luck was in, as Laporte could not sort his feet out in time and inadvertently found his own net from six yards, bringing St Mary’s to its feet.
In the last round against West Ham, Hasenhüttl made a double change at the interval despite his team holding a half-time lead, later revealing the switch was premeditated.
On this occasion, despite having five substitutions at his disposal, the boss was happy to stick rather than twist – at least for the time being.
Adam Armstrong had the first sight of goal after the restart, dragging a low shot wide from 20 yards. The striker has a habit of getting opportunities, but was just missing a clean connection when it mattered most.
The same was true soon after, when James Ward-Prowse intercepted a De Bruyne pass and immediately sent Armstrong on his way, with only Walker to beat.
Unfortunately the defender is about as quick as they come, but Armstrong was intelligent and waited for the right moment to dart inside and shoot – this time seeing a deflection take the ball wide of goal for a corner.
Another Ward-Prowse interception on the hour presented another half-chance, and the skipper struck it well, low to Steffen’s right, but the American was down sharply and his handling was faultless.
The game would swing back in City’s favour when Salisu was penalised for bringing down Jesus just inside the penalty area – a soft decision that left the home team feeling hard done by.
Up stepped De Bruyne, who was rather fortunate to see his kick squirm beyond Forster’s outstretched right hand and creep into the bottom corner.
Hasenhüttl would have been doubly frustrated having prepared the introduction of Broja and Adams, who were now forced to enter the fray with Saints trailing, as Long and Adam Armstrong departed on 63 minutes.
As the clock ticked down into the final 20 minutes, Saints had to push, which left gaps for City to exploit on the counter-attack, leading to a scuffed volley wide from Mahrez.
The big chance to equalise fell to Ché Adams 17 minutes from time. Ward-Prowse’s brilliant switch of play released Elyounoussi, who took the ball superbly in his stride and drove towards goal, squaring for Adams, but his shot was too close to Steffen who made a vital save.
At the same time that Hasenhüttl had brought on Broja and Adams, Pep Guardiola had made two changes of his own that were about to take the game away from Saints.
First Foden let fly with an unstoppable shot that arrowed into the corner to beat the helpless Forster from 20 yards, underlining the youngster’s potential to be a superstar in the years to come.
Three minutes later, fellow substitute Mahrez gave the scoreline a very harsh reflection when he fooled Forster with a low shot inside his near post 12 minutes from time.
Saints did not deserve such a result, but it was a harsh reminder that chances must be taken and mistakes will be punished against the best in the business.