John Terry, who will leave the Blues at the end of the season, opened the scoring in his last game at the Bridge, before the defender made a mistake leading to Etienne Capoue’s equaliser just over a minute later.
César Azpilicueta and Michy Batshuayi responded for the hosts either side of half-time, and Antonio Conte’s side looked in control.
Daryl Janmaat pulled one back straight away and Stefano Okaka levelled soon after, but Fàbregas drove home with three minutes to go to snatch all three points.
Terry involved in goals at both ends
Neither side managed to create a real opening in the game, with Watford struggling to get into the final third of the pitch before the hosts broke the deadlock.
Willian played a corner into the penalty area, and after the ball bounced off the head of Kurt Zouma and off Terry’s face, the Chelsea skipper reacted and slotted the ball home off the post – the 100th goal scored by the Blues in all competitions this season.
The Blues hadn’t been threatened by Walter Mazarri’s game before the goal, but were gifted a goal at the other end just over a minute later.
It was an up and down minute for Terry, who headed the ball up into the air in the penalty area, which allowed Capoue to latch onto the mistake and finish past Asmir Begović.
The midfielder netted Watford’s first goal in four games, and their first goal away from Vicarage Road since January.
Azpilicueta restores Chelsea's lead
Conte’s side went close when Eden Hazard fired just past the post just after the half-hour mark, and four minutes later, they went back ahead.
Hazard’s corner was cleared out to Azpilicueta on the edge of the area, and the Spaniard hammered the ball past Heurelho Gomes.
The home side came close in the final minutes of the first-half when Kenedy fired wide, but Conte’s side took a deserved lead into half-time.
Chelsea didn’t have to wait long to double their advantage in the game, with the Blues making it 3-1 just four minutes after the restart.
Janmaat responds for Watford after Batshuayi's goal
Kenedy’s deflected pass went into the path of Nathan Ake, whose pass back across goal was tapped home by Batshuayi – the striker netting for the second successive game.
However, history repeated itself as the Hornets managed to reply just over a minute after the restart, just as they did for the first Chelsea goal.
Janmaat picked up the ball and burst down the flank, the defender cut back inside and drove into the far corner past Begović.
The next opening came on 66 minutes when Kante fired a cross into the feet of Batshuayi, who scuffed his shot into the arms of Gomes.
Blues snatch all three points through Fàbregas
Watford’s ‘keeper was called into action once more as he had to push Azpilicueta’s shot around the post before the Hornets found an equaliser three minutes later.
Tom Cleverley’s ball into the box was nodded down by Abdoulaye Doucoure, and substitute Okaka, who had only been on the pitch a matter of minutes, latched onto the loose ball before thrashing the ball home.
The visitors looked the more likely to go on and find a winner as Jose Holebas’ pass found Janmaat, who struck an effort just over the bar – Begović was at full stretch.
Chelsea responded and, once again, went back ahead in the game as Willian’s pass found Fàbregas, who volleyed home past Gomes.
There was still enough time for more drama as Sebastian Prödl received his marching orders for a second late tackle in the game.
The Blues held on for all three points and will get their hands on the title on Sunday as they welcome Sunderland to Stamford Bridge.