Mason Mount scored Chelsea's first goal and set up the winnerChelsea were "lucky" to remain at the top of the Premier League after being given a scare in their victory at Watford, said boss Thomas Tuchel.
The under-par Blues needed a goal from substitute Hakim Ziyech to edge out their hosts after Hornets forward Emmanuel Dennis had cancelled out Mason Mount's opener.
With title rivals Manchester City and Liverpool both victorious, Chelsea would have slipped to third had they drawn - but Ziyech ensured they still lead the way.
"We were not ready for this game," said Tuchel.
"We are to blame. We never found the right attitude. We were very lucky to escape with the win."
Watford had twice gone close to taking the lead when England midfielder Mount broke the deadlock with a first-time finish at the end of a lovely move involving Marcos Alonso and Kai Havertz.
The Hornets equalised after Ruben Loftus-Cheek lost possession and Moussa Sissoko set up Dennis, who scored with a shot that deflected off Antonio Rudiger.
But having scored his side's first, Mount then set up the winner by providing the pass for Ziyech to fire past Daniel Bachmann.
Chelsea's fifth straight Premier League away win kept them one point clear at the top.
The game was halted for 32 minutes with the score 0-0 while a fan, who had suffered cardiac arrest in the stands, was stabilised.
Chelsea were 'lucky' to get win at Watford - TuchelChelsea dig deep to stay top
In a bruising contest which featured seven yellow cards, Chelsea were a long way from their best.
But Tuchel's side found a way to secure a hard-fought three points and maintain their advantage at the summit.
Even before the game was stopped in the 12th minute, Watford had twice gone close. But then Mount - who had earlier hit the post - swept the visitors into the lead.
Dennis' fourth goal in five league games arrived shortly before half-time to lift the hosts, and it could have been even better for them.
Tom Cleverley found himself in a one-on-one with Edouard Mendy but slipped at the crucial moment after the Chelsea keeper came rushing out of his box.
It proved a costly miss as Ziyech, sent on after Trevoh Chalobah was injured, condemned Claudio Ranieri's side to a sixth defeat in eight league games.
However, there were positives in defeat for Ranieri who, after last month's win over Manchester United, saw another spirited home performance from his players.
"We must continue this way and sooner or later the points will arrive," said the Watford boss, whose side remain 17th in the table and host second-placed Manchester City on Saturday.
Watford boss Ranieri 'proud' despite Chelsea defeatPlayer of the match
HavertzKai Havertz
Chelsea
Squad number29Player nameHavertz
Squad number22Player nameZiyech
Squad number19Player nameMount
Squad number16Player nameMendy
Squad number6Player nameThiago Silva
Squad number4Player nameChristensen
Squad number9Player nameLukaku
Squad number10Player namePulisic
Squad number2Player nameRüdiger
Squad number14Player nameChalobah
Squad number28Player nameAzpilicueta
Squad number12Player nameLoftus-Cheek
Squad number3Player nameAlonso
Squad number17Player nameSaúl
Line-ups
Watford
Formation 4-1-4-1
26Bachmann
21Femenía5Troost-Ekong15Cathcart11Masina
6Louza
25Dennis8Cleverley19Sissoko10João Pedro
7King
26Bachmann21Femenía5Troost-EkongBooked at 45mins15Cathcart11MasinaSubstituted forRoseat 12'minutesBooked at 84mins6LouzaBooked at 45minsSubstituted forKuckaat 73'minutes25Dennis8CleverleyBooked at 84minsSubstituted forNgakiaat 85'minutes19Sissoko10João Pedro7KingSubstitutes
2Ngakia3Rose16Gosling17Fletcher18Tufan27Kabasele29Hernández33Kucka35ElliotChelsea
Formation 3-4-2-1
16Mendy
14Chalobah4Christensen2Rüdiger
28Azpilicueta12Loftus-Cheek17Saúl3Alonso
19Mount10Pulisic
29Havertz
16Mendy14ChalobahSubstituted forZiyechat 60'minutesBooked at 65mins4Christensen2Rüdiger28AzpilicuetaSubstituted forLukakuat 69'minutes12Loftus-Cheek17SaúlBooked at 16minsSubstituted forThiago Silvaat 45'minutes3AlonsoBooked at 45mins19Mount10Pulisic29HavertzSubstitutes
1Arrizabalaga5Jorginho6Thiago Silva9Lukaku11Werner18Barkley20Hudson-Odoi22Ziyech31SarrReferee:David CooteAttendance:20,388Live Text
Match ends, Watford 1, Chelsea 2.
Second Half ends, Watford 1, Chelsea 2.
Corner, Watford. Conceded by Marcos Alonso.
Attempt blocked. Kiko Femenía (Watford) right footed shot from outside the box is blocked.
Ruben Loftus-Cheek (Chelsea) wins a free kick in the defensive half.
Foul by William Troost-Ekong (Watford).
Corner, Watford. Conceded by Édouard Mendy.
Attempt saved. Juraj Kucka (Watford) right footed shot from outside the box is saved in the top centre of the goal.
Foul by Christian Pulisic (Chelsea).
Emmanuel Dennis (Watford) wins a free kick in the attacking half.
Corner, Watford. Conceded by Marcos Alonso.
Corner, Watford. Conceded by Hakim Ziyech.
Attempt blocked. Juraj Kucka (Watford) right footed shot from outside the box is blocked. Assisted by Joshua King.
Ruben Loftus-Cheek (Chelsea) wins a free kick in the defensive half.
Foul by Jeremy Ngakia (Watford).
Substitution, Watford. Jeremy Ngakia replaces Tom Cleverley.
Danny Rose (Watford) is shown the yellow card for a bad foul.
Tom Cleverley (Watford) is shown the yellow card.
Attempt saved. Hakim Ziyech (Chelsea) header from the centre of the box is saved in the centre of the goal. Assisted by Kai Havertz with a cross.
Offside, Watford. Emmanuel Dennis tries a through ball, but Tom Cleverley is caught offside.
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