Chelsea came out 2-1 victors at Vicarage Road despite an underwhelming performance against Claudio Ranieri's Watford.
Mason Mount opened the scoring in the first-half after a great move involving Marcos Alonso and Kai Havertz.
Emmanuel Dennis equalised on the brink of half-time, taking advantage of a Ruben Loftus-Cheek mistake.
The Blues did not play well but found the winner through substitute Hakim Ziyech, rushing onto a Mount cross to fire Chelsea ahead.
Thomas Tuchel's men were penned in in the early stages, with Edouard Mendy pulling off a fantastic save to keep out a cross which deflected off Cesar Azpilicueta.
The game was stopped moments after due to a medical emergency as a fan in the crowd suffered a cardiac arrest.
The players left the pitch before returning later on to resume the first-half.
Chelsea's first chance came 18 minutes in and Mount should have put the Blues ahead. Lofus-Cheek's header fell to his English team-mate but a volley smashed off the woodwork and back into play.
However, Mount made up for it as he put Chelsea ahead on the half an hour mark. A fantastic pass from Antonio Rudiger found Alonso, whose pull-back to Havertz allowed the German to take a touch and find Mount alongside him for an easy finish.
Minutes later, Havertz had the ball in the back of the net himself as Christian Pulisic sent him through on goal. Unfortunately, the American's pass was too late and Havertz had strayed into an offside position before a lovely left-footed finish.
Mendy kept his side infront with 10 minutes remaining of the half, denying a Danny Rose strike.
However, Chelsea could not hold onto the lead going into half-time as Emmauel Dennis added his sixth goal of the season.
Loftus-Cheek lost the ball in midfield, allowing the Hornets to counter-attack before Dennis picked the ball up and picked out the bottom left corner, with aid of a Rudiger deflection.
The Blues came out playing better in the second-half as Saul Niguez was replaced by Thiago Silva and Trevoh Chalobah moved into central midfield.
There were several injury concerns in the first 15 minutes of the second-half as Mendy rushed Tom Cleverley, who was in on goal. The goalkeeper did well to keep put the midfielder off, but was caught.
In the same passage of play, Chalobah went down with what looked to be an impact injury and was replaced by Hakim Ziyech.
Havertz looked the most impressive Chelsea player as, with 70 minutes on the clock, the German picked the ball up and ran across the pitch with the ball, breaking into the Watford box before unleashing a deflected shot into the goalkeeper's hands.
However, Tuchel's side were struggling to score a second as Romelu Lukaku replaced Azpilicueta.
Just moments after Lukaku's introduction, Chelsea were ahead against the run of play. Lukaku's run into the box freed up space for Ziyech, who ran into the box to fire home a Mount cross.
Lukaku's impact was clear as the striker began taking on the Watford defenders and giving them a physical battle late in the match.
The Moroccan Ziyech could have added a second, meeting a cross unmarked in the box but his header could not beat Daniel Bachmann.
Chelsea sat deep in the final minutes as they held on to come away with a valuable three points to stay top of the Premier League.