Antonio Conte made nine changes to his starting line-up and the Blues launched the first attack of the match but, after Willian had created the opening, both Michy Batshuayi and Eden Hazard had shots blocked.
However, Chelsea took the lead midway through the first half when John Terry’s close-range shot went in off a post.
Watford’s response was immediate though and they equalised in the 24th minute when Terry’s error let in Etienne Capoue who nodded the ball over Asmir Begovic.
The lively Willian had a shot blocked by Sebastian Prodl and Hazard went close to scoring before Chelsea went ahead for a second time in the 36th minute when, from a corner, Cesar Azpilicueta fired a low shot past a crowd of players and into the back of the net.
At the start of the second half Chelsea scored again when, from Willian’s corner, Nathan Ake found Batshuayi who tapped the ball in for his second goal in two games.
Watford responded quickly again though with Daryl Janmaat cutting inside Ake before firing his shot past Begovic, and they made it 3-3 with 16 minutes left when Tom Cleverley found substitute Stefano Okaka who fired his 12-yard shot home.
However, with just two minutes left N’Golo Kante set up Fabregas to score the winner before Prodl was sent off in stoppage time for a second bookable offence.