Watford 1-2 Southampton
Brett Ormerod proved the match-winner as Southampton ended Watford's FA Cup run with victory at Villa Park.
Ormerod scored two minutes before half-time to end a run of 23 games without a goal for the Saints.
And the forward set up the crucial second goal that put the Saints into the final and all but secures them a place in Europe next season.
With final opponents Arsenal virtually guaranteed a place in Champions League the Saints are likely to take the Uefa Cup place.
Marcus Gayle scored late on to give Watford hope, but Gordon Strachan's men held on against their plucky opponents.
Watford began brightly and proved to be a tough test for their Premiership opposition.
The Hornets' on-loan forward Michael Chopra caught the eye early on with some nice touches, but it was strike partner Heidar Helguson who nearly opened the scoring.
Neil Ardley floated in a free-kick and Helguson found himself free at the back post.
His header appeared to hit James Beattie on the arm and deflected towards goal but Saints keeper Paul Jones saved with his feet.
Gayle's goal came too late
43 mins: Ormerod opens the scoring for Saints
78 mins: The unfortunate Robinson scores an own goal
86 mins: Gayle's header gives Watford hope
After a slow start Southampton began to enjoy more of the possession with Beattie looking their best chance of a breakthrough.
He headed over from a Wayne Bridge cross and then latched onto a long pass that was misjudged by Watford defender Neil Cox.
The ball fell into the path of the in-form forward but he could only get his toe to it and it fell the wrong side of the post.
When the first goal came it was Beattie's strike partner Ormerod who was the scorer.
Chris Marsden created the chance with a fine run and cross on the left flank and Ormerod headed home from close range for his first goal since October and first away from St Mary's this season.
It was the first goal Watford had conceded in the competition this season.
Southampton should have doubled their lead early in the second half when Beattie crossed for Michael Svensson, but he could only head straight at keeper Alec Chamberlain.
Watford showed they were not going to go down without a fight and Bridge had to make a last-ditch tackle to prevent Chopra from a clean shot on goal.
Micah Hyde then side-footed wide and Glass headed against the crossbar as the Hornets threw everything at their opponents.
But with 10 minutes left on the clock Ormerod crossed and with Beattie looking for the tap-in, Watford defender Paul Robinson bundled the ball past his own keeper.
Gayle's header gave the Saints some hope but they were unable to continue the fairytale.
Southampton will go to Cardiff as massive underdogs, much as they did on their last FA Cup appearance in 1976, when they beat Manchester United to lift the trophy.
Watford: Chamberlain, Mahon, Cox, Gayle, Robinson, Ardley,
Hyde, Vernazza (Nielsen 56), Glass (Cook 77), Helguson,
Chopra (Tommy Smith 70).
Subs not used: Dyche, Lee.
Southampton: Jones, Telfer, Lundekvam, Michael Svensson,
Bridge, Fernandes, Anders Svensson (Delap 78), Oakley, Marsden,
Ormerod (Tessem 90), Beattie.
Subs not used: Davies, Higginbotham, Blayney.
Referee: M Riley (W Yorkshire).