Birmingham City's unbeaten start to the Championship season came to an end as two stoppage-time goals from Mileta Rajovic and Ryan Andrews gave Watford their first win since the opening day.
In-form Dane Rajovic, who scored twice on his full debut in their 3-3 draw against Coventry City just before the international break, thumped his effort beyond John Ruddy in the 91st minute to put the Hornets in front.
Andrews then pounced to fire in his first Watford goal six minutes later to punish a tiring Blues, who had Lee Buchanan sent off for a second booking late on.