Hakim Ziyech’s winner kept Chelsea at the top of the Premier League.
Mason Mount put them in the lead at Vicarage Road, where the game was suspended for almost half an hour in the first half because a fan suffered cardiac arrest – but it was later said that he was stable.
It was a tough night for the Blues against a side from Watford who got them into trouble and equalized by Emmanuel Dennis a few minutes before halftime.
Mount, having already shot against the post, marked his return to the starting lineup with the first goal.
Marcos Alonso did well on the left and chose Kai Havertz, who played the ball to his right to hit Mount for the emphatic first time.
But Watford, much improved since the arrival of former Chelsea boss Claudio Ranieri, has rightly equalized.
Ruben Loftus-Cheek was at fault, losing the ball to Moussa Sissoko, who found Dennis.
The Nigerian striker faced Antonio Rudiger and his shot in the legs of the defenders deflected him slightly, leaving goalkeeper Edouard Mendy helpless.
Thomas Tuchel has made six changes, including giving Saul his first Premier League start since September, when he was substituted at half-time after having had a bad debut.
And it was the same result this time around – the midfielder was again substituted in the interval after also struggling to keep up with the pace of the game.
Thiago Silva was sent in his place, Trevoh Chalobah moved to midfield and Chelsea were better in the second half, although still not at their best.
The decisive goal, scored 18 minutes from time, was well worked out.
Alonso directed Rudiger’s pass to Mount, who galloped to the left and crossed for Ziyech, as a substitute, to score from eight yards.
Chelsea: Mendy, Azpilicueta (Lukaku 69), Chalobah (Ziyech 60), Christensen, Rudiger, Alonso, Loftus-Cheek, Saul (Silva 45), Mount, Pulisic, Havertz
Unused Submarines: Kepa, Jorginho, Werner, Barkley, Hudson-Odoi, Sarr.