It had hardly been a wobble for Manchester City. A couple of 0-0 draws, the first against Sporting in a Champions League tie already sewn up; the second at Crystal Palace in the Premier League. But when the club give their critics so little ammunition and their rivals so little hope, anything will be seized upon – particularly with Liverpool so remorseless, hoovering up the space behind them in the league. This was a brutal flexing of City’s authority, a step into a fifth FA Cup semi-final in six seasons under Pep Guardiola, the manner of their surge to victory in the closing stages taking the breath. Southampton had been game opponents, putting City on the back foot at times, and the substitute Ché Adams had a golden chance for 2-2 in the 74th minute only to shoot too close to Zack Steffen after good work from Mohamed Elyounoussi. City responded as if affronted. Guardiola had introduced Phil Foden and Riyad Mahrez as substitutes on 63 minutes. Now they turned the screw. Foden’s goal for 3-1 was a shot of vicious power, the ball exploding off his left boot from the edge of the area to zip past Fraser Forster and, when Mahrez lashed inside the near post shortly after, some Southampton fans made for the exits. They missed Mahrez and Raheem Sterling blowing glorious late chances as their team fell apart. It is one thing to match the City machine for the majority of a contest. Quite another to do so over the duration. Southampton’s dream was of a third FA Cup semi-final appearance in five seasons. City laid it waste. There were two turning points, Adams’s miss being the second. The first had come just after the hour at 1-1 when Mohammed Salisu stretched into a tackle on Gabriel Jesus that he did not have to make. The danger was not pronounced as Jesus ran up the right side of the penalty area and, when the City forward pulled a stop-and-go move, feeling the contact from Salisu, it was as if the air had been sucked out of the stadium. Kevin De Bruyne, excellent throughout, jammed the kick into the bottom corner, Forster narrowly failing to get a hand down in time. Southampton had held City to a pair of draws in the league this season but they made too many mistakes now, with Jack Stephens guilty of a bad one in the 12th minute which led to the opening goal. The centre-half got his feet into a tangle, fluffing an attempted clearance from inside his area straight to Jesus, who moved the ball to Sterling. The England forward had previously sliced when well-placed. Now he buried the shot with the aid of a deflection off Tino Livramento. For Southampton, the margins were tight. Moments earlier, Adam Armstrong had hit the far post after a fine Oriol Romeu through ball. Now they had it all to do. City had won all 27 of the previous matches this season in which they had taken the lead. These are discombobulating times, geopolitics casting a shadow over the sport we love, and the City support has had to process the deeply uncomfortable sight of the club’s owner, Sheikh Mansour bin Zayed al-Nahyan, welcoming the Syrian president, Bashar al-Assad, to the United Arab Emirates on Friday. Chris Bryant MP has questioned whether Mansour is a “fit and proper person to be owning a football club”, saying it would be “good to see the back of him”. The game goes on. It always does. City ought to have scored again on 27 minutes when João Cancelo beat Livramento to cross for Ilkay Gündogan only for his first-time effort to come back off a post. Rodri also whistled a shot wide from distance. But City were disappointing in the last 15 minutes of the first half, Guardiola saying that they lacked the personality to play, and Southampton got the equaliser when Steffen left his line to confront Elyounoussi, who had chased a Stuart Armstrong pass up the inside right. Steffen panicked and retreated, caught in a kind of no man’s land and, when Elyounoussi went for the cut-back, the ball hit Aymeric Laporte and spun into the net for an own goal. It was hugely fortunate for Southampton but reward for their intensity and positivity. They had also flickered on 24 minutes when Adam Armstrong snatched at a presentable shooting chance. City were better in the second half, Mahrez spurning a volleyed opportunity after De Bruyne’s penalty and, when Adams could not finish, they moved up through the gears, Guardiola ending the day being asked about the Treble. “How many times has it happened in this country?” he said. “Once. It’s not easy.”