Chelsea kept their place at the top of the table but had to work hard for victory against a battling Watford who have improved hugely under former Blues coach Claudio Ranieri.
In a game disrupted for over half an hour by a medical emergency when a Watford supporter needed treatment in the stands after suffering cardiac arrest, Thomas Tuchel had to make a number of tweaks before Hakim Ziyeh scored the winning goal to keep Chelsea top ahead of Manchester City and Liverpool, who are both hot on their heels after victories of their own.
Mason Mount had put Chelsea ahead before Emmanuel Dennis equalised, after a 35-minute break in play.
Barely 12 minutes had passed when Adam Masina went down for treatment on Watford's left wing, but players began gesticulating to the referee and benches that an emergency situation was developing on the other side of the pitch, in the upper tier of the Graham Taylor stand.
Medical staff from both clubs rushed over to join the stewards and supporters helping the stricken fan, who had suffered a cardiac arrest, it emerged later. Both sets of players went back to their dressing rooms, and over half an hour passed before they emerged again, after it was announced that the fan had stabilised and been taken to Watford General Hospital, conveniently situated behind the stand.
When play got underway, the pattern of the first 12 minutes was repeated, with Watford well on top against an uncharacteristically nervous-looking Chelsea side. Tuchel's defence looked particularly shaky without Thiago Silva, one of six changes from the side that drew with Manchester United on Sunday.
Andreas Christensen is expected to sign a new contract soon, but he looked a pale imitation of his Brazilian team-mate. It was no great surprise to see Thiago introduced at half-time, at the expense of Saul, who had another miserable 45 minutes before being hooked.
But in those opening stages, no one in the Blue of Chelsea did themselves proud. Antonio Rudiger and Trevoh Chalobah were also guilty of causing problems for their goalkeeper Edouard Mendy, who was making uncharacteristic mistakes himself.
He was by far the busier keeper for most of the first half-hour, keeping out shots from Dennis and Danny Rose as well as preventing Cesar Azpilicueta from scoring an own goal.
Chelsea could hardly get the ball into the Watford half for 29 minutes, so it was completely against the run of play when Mount put them ahead with their first attempt on goal. Rudiger chipped a forward pass into the path of Marcos Alonso, who cut the ball back from the left. Kai Havertz had a chance to score, but unselfishly laid it into the path of Mount, who sidefooted the ball firmly past Daniel Bachmann.
Watford were not deterred. Driven on by Moussa Sissoko, they kept going at Chelsea, and it was the former Tottenham man who set up the equaliser. Sissoko won the ball in the centre circle and picked out Dennis on the right. The Nigerian has been a big hit since arriving at Vicarage Road in the summer and showed why with a classy goal, shimmying left and right to bamboozle Rudiger before firing a low shot that clipped the German defender's heel before flying into the far corner of goal.
There was almost a second in first-half stoppage time as Sissoko exchanged passes with Dennis before firing weakly at Mendy from close range, and Watford left the field aggrieved not to be ahead at the break.
Tuchel made his first change by hooking the hapless Saul and changing shape, with Silva going to centre-back and Chalobah moving into midfield, although he went off injured on the hour mark, to be replaced by Ziyech. Christian Pulisic was put in on goal by Ruben Loftus-Cheek but failed to get a shot away before being crowded out, and Havertz had a shot saved by Bachmann at his near post.
Watford had chances too, and Mendy had to come flying 15 yards out of his penalty area to foil Tom Cleverly as the former Manchester United midfielder ran towards goal.
Tuchel had seen enough and sent on Romelu Lukaku with 22 minutes remaining, changed his team's shape yet again, and Chelsea regained the lead soon afterwards.
A long ball forward found Mount on the left, the young England attacker crossed low into the path of Ziyech, who hit a first-time shot without breaking stride into the roof of the net from 12 yards. It was the Moroccan's first league goal of the season, and he almost had another ten minutes later when he met a Havertz cross with a thumping header, only to see Bachmann save on the line.
Even in stoppage time, Watford kept going and Mendy had to tip a rasping drive from substitute Juraj Kucka over the bar.
It was certainly not a comfortable win for Tuchel, but his side remain top of the table.
Bachmann 7; Femenia 6, Troost-Ekong 7, Cathcart 7, Masina 6 (Rose 12); Louza 7 (Kucka 73); Dennis 8, Sissoko 7, Cleverley 7 (Ngakia 85), Joao Pedro 7; King 6.
Mendy 6; Chalobah 6 (Ziyech 60) , Christensen 6, Rudiger 6; Azpilicueta 6 (Lukaku 68), Loftus-Cheek 7, Saul 5 (Silva 46), Alonso 6; Mount 7, Havertz 7, Pulisic 6.
D Coote 4.