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Southampton, which upset Arsenal 2-1 in the quarter-finals, looked poised to continue its dream run when Finn Azaz struck a gorgeous shot from distance in the 79th minute that James Trafford had little chance of stopping.

But in a thrilling second half of the semi-final, Jeremy Doku wiped out Southampton's lead four minutes later when he worked the ball on to his right foot before unleashing a shot from the edge of the penalty area that deflected off James Bree and into the net.

Gonzalez struck a rocket from 30 yards out, the longest goal scored by any City player this season, in the 87th minute and City, which won the Carabao Cup earlier this season and is neck and neck with Arsenal at the top of the Premier League table, held on for victory in a breathless final few minutes.

"Really special for me, especially because I'm not used to scoring these goals," Gonzalez said after the match. "So with the winning goal like this, in this amazing stadium, it's amazing."

City, which has hoisted the FA Cup trophy seven times, will meet the winner of Monday's (AEST) semi-final between Chelsea and Leeds United in the 17 May (AEST) final at Wembley.

"Every time you reach the final you're like wow what a journey it has been," Doku said. "Reaching the final again is unbelievable.

"I was analysing the game in the first half. We were playing in the middle a lot. I knew that I had to bring a threat and make it uncomfortable for the fullbacks. I watched [Southampton's] game against Arsenal and I felt they deserved to win. They have a lot of quality players, I was not surprised [by how good they were]."

Southampton, which is 23 places below City in England's football pyramid in fifth in the second-tier Championship, was the better team in the first half against what was virtually a second-string City line-up, and had a goal from Leo Scienza chalked off for offside.

But Pep Guardiola's side was significantly better after the break, and a couple of key changes including sending on substitutes Erling Haaland and Doku, made a huge difference as City finished with 24 shots to Southampton's four, and six shots on target for the victors to three for the Saints.

The victory was City's 22nd in a row against FA Cup opponents from lower-division teams, but Southampton made the Premier League titan dig deep for it.

"It's the FA Cup, semi-final is always like that," Guardiola said. "We make an an incredible, incredible, incredible second half, really really good.

"It's always nice to be here fighting against top teams. No team has made four finals in a row. It's extraordinary and hopefully we can arrive with a good momentum."

The loss ended Southampton's 20-game unbeaten run across all competitions.

"It's emotional at the end but the reality is, just like every other game, we need to put this one to bed as quick as possible," manager Tonda Eckert, whose team are in the thick of the fight for Premier League promotion play-off spots, said.