The Ghana international smashed in a thunderous strike from inside the box into the roof on 82 minutes.
Ayew had missed a penalty in the 64th minute and needed that goal to placate the fans as well as make amends.
The 30-year-old has now scored 16 league goals- his highest tally ever in any league.
Brentford were reduced to 10 men after Rico Henry was given his marching orders for a two-footed challenge on Connor Roberts.
Ayew picked the ball but goalkeeper Raya guessed correctly to deny him from 12 yards.
Brentford will host the Swans on Wednesday in the return leg to decide the winner of the other semi-final between Fulham and Cardiff. Read Full Story
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