The striker got between two Watford centre-backs and races through on goal, getting to the ball before Foster and firing it over his head and into the back of the net.
55 minutes on, Pulisic made it 2-0 for Chelsea. Willian got the ball in the middle and lays it out wide to Abraham, who drills it into the six-yard box for Pulisic to tap it home from just a couple of yards out.
80th minutes, VAR overuled the original decision by the referee after a lengthy delay to give Watford a penalty.
Deulofeu stepped up and converts the penalty to makes it 2-1. The winger sent Kepa the wrong way and coolly slots the ball into the back of the net.
The Blues now move third position in Premier League table while waiting for the outcome of Leicester City and Crystal Palace game tomorrow.