Etienne Capoue put Watford ahead after 55 minutes with his second goal of the season, but Michy Batshuayi equalised in the 80th minute with his first goal for Chelsea, six minutes after coming off the bench, thanks to an error by Watford goalkeeper Heurelho Gomes.

And Spain striker Costa won it with three minutes to go at Vicarage Road after a fine pass from another substitute, Cesc Fabregas.

If Fabregas had a point to make after failing to make Conte's starting line-up for either of the first two matches, then he may consider it made, having also played a part in Batshuayi's equaliser.

Conte and Mazzarri, who had a frosty relationship when they managed Juventus and Napoli respectively in Italy, are animated performers in the technical area and not even some unseasonal rain put them off as they both spent most of the match on their feet.

But Conte won the battle in the end, even if a draw might have been a fairer result on the balance of play.

Watford paraded new signings Roberto Pereyra, a £13 million ($17 million, 15 million euros) capture from Juventus, and Isaac Success before the match. 

They and Younes Kaboul, who joined on Friday from Sunderland, may reflect that their extra quality might convert results such as this into better outcomes for their new team.

Jose Holebas had the first serious shot at goal after seven minutes, advancing from his left wing-back position to meet a diagonal pass with a low angled half-volley that goalkeeper Thibaut Courtois had to block with his feet.
                  
- Bullied -                  
                  
And the other wing-back, Nordin Amrabat, bullied his way past Cesar Azpilicueta and curved in a dangerous cross that Odion Ighalo, sliding in along the rain-slicked turf, could only hit over the bar.

Amrabat then half-volleyed a decent chance high and wide after Chelsea had half-cleared a cross from the left.
And the Dutch-born Moroccan international was a central player again as Watford took the lead 10 minutes into the second half. 

Amrabat's pass found Adlene Guedioura and his cross from the right swung behind forwards and defenders alike but reached Capoue beyond the far post.

He had time to chest the ball down and strike a fierce left-foot half-volley high past the right hand of Courtois. 
The French midfielder had not scored for Watford since joining the club in July 2015 until he found the net at Southampton last week. 

Chelsea tried to react but Costa, already on a booking, fooled nobody, including referee Jon Moss, when he appeared to dive after a challenge by Miguel Britos in an unsuccessful bid to win a foul.

Conte sent on Spain midfielder Fabregas in an attempt to bring some accuracy to his team's passing, and it worked. 

He found Eden Hazard and his shot was fumbled by Gomes to Batshuayi, who could not miss from six yards.

Then, in the 87th minute, Fabregas' diagonal pass sent Costa racing clear to slip the ball under Gomes for his second late winner in a week following Monday's decisive strike against West Ham United.

Batshuayi could even have made it 3-1 but his shot from 12 yards came back off the crossbar.