The short corner was pulled back to Pedro who curled a fantastic strike in off the top left hand corner of the goal frame.
The Blues came back from behind to win for the first time this season, moving them up above their opponents and into fourth place.
But while that goal got the Blues off to a ideal start, having been trailing league leaders Manchester City by nine points going into the game, things turned sour seconds later.
Chelsea failed to properly clear a free kick and it went out for a throw-in which Jose Holebas launched into the area.
Watford manager Marco Silva has complained of a "really unfair result" after seeing his side lose out 4-2 to Chelsea at Stamford Bridge this afternoon. David Luiz attempted to head the ball out but it dropped down for Doucoure to fire home through a crowd of players.
"We played our game and in the second half for 40 minutes there was one team playing on the pitch".
It could have been worse for Antonio Conte's side as Brazilian Richarlison missed two glorious chances for Watford either side of Pereyra's goal.
The champions then took the lead in the 87th minute when an un-marked Cesar Azpilicueta headed home Willian's cross at the far post.
There was still time for a fourth in stoppage time as Britos conceded possession and Batshuayi pounced to round off the victory.
Watford come into this sitting higher in the table than Chelsea, who now sit fifth in the Premier League.
Chelsea struck first in the 12th minute with a right-footed shot by Pedro from outside the box, assisted by Eden Hazard following a corner. The hosts drew back in the 71st minute with a header by Michy Batshuayi following a set piece.
For large parts of the first period, however, Watford were the ones in control and it was no more than they deserved when Doucoure equalised just before the break.