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.
Birmingham's own in-form striker Jay Stansfield had Blues' best moments, forcing a smart save from Daniel Bachmann after 45 seconds and then hitting the post with a fine effort in the second half.
Scott Hogan was also denied twice within seconds by Bachmann as John Eustace's side were left rueing the chances that slipped away.
Both sides created early opportunities in a competitive first half in the Hertfordshire sunshine as Stansfield, hunting a third goal in as many league games, swapped passes with debutant Oliver Burke down the right to warm the hands of Bachmann with a low drive inside the first minute.
Watford then almost took the lead when Scotland defender Ryan Porteous saw his shot from a tight angle strike the base of Ruddy's post following a free-kick.
Burke continued to be bright as the two teams sized each other up as the half wore on and, despite Watford enjoying the bulk of possession, Tom Dele-Bashiru's effort from distance that flew over was the best they could muster.
Fulham loanee Stansfield almost broke the deadlock on the hour as he collected Dion Sanderson's pass before seeing his splendid right-foot curling shot beat the stretching Bachmann but cannon off the left-hand post to safety.
Bachmann then kept his side in it with a thrilling double save from Hogan - first with his arm, then his foot, as Blues pushed hard to extend their excellent start to the campaign.
Blues' quest to guarantee a point was hit when Buchanan picked up his second yellow card for holding Yaser Asprilla back.
And it was Asprilla who then created Watford's first goal as Rajovic powered home a header from his cross as the game entered 10 minutes of added time.
The Hornets then put the result beyond doubt five minutes later when Andrews drove home his first senior goal from the edge of the box.
Victory lifts Watford into the top half with Blues still in the top six.
Watford head coach Valerien Ismael told BBC Three Counties:
"I'm very pleased for Mileta [Rajovic] - it was a tough game for him but in the end he got the reward.
"We know Danny [Bachmann} makes big saves and today he made a big difference for us to keep a clean sheet and then get the job done.
"Patience was the key to the game for us and everyone got the reward for showing that.
"We should have more points on the board but today was important for the mental side of our game and it should give the players confidence."
Birmingham City boss John Eustace told BBC Radio WM:
"I thought we were outstanding - I think we limited probably the best squad in the league to very little.
"We didn't deserve to lose the game and I think with 11 men on the pitch, we don't lose it.
"I think probably is a double yellow [for Lee Buchanan] in the end, but I think the inconsistency throughout the game made it worse that what it was.
"I still expected us to defend the last six minutes with 10 men - we had defended our box really well all game and looked a threat on the turnover.
"We had a couple of good chances ourselves - it's a tough one to take."