Watford 1 Chelsea 2: Batshuayi and Costa leave it late for Conte's men
Diego Costa and Michy Batshuayi scored twice in the final 10 minutes to secure a 2-1 victory over Watford and maintain Antonio Conte's unbeaten start as Chelsea manager.
Batshuayi pounced seven minutes after being introduced at Vicarage Road as Heurelho Gomes failed to hold an Eden Hazard shot, before Costa tucked home the winner.
The Spain international raced clear with three minutes to play and coolly fired underneath Gomes to seal all three points, and extend Conte's undefeated run against Walter Mazzarri to seven league games.
The Italian coaches fell out in 2011 after Conte was deemed to have copied Mazzarri's 3-5-2 formation for a Juventus clash with Napoli, and they have endured a frosty relationship ever since.
Etienne Capoue's second strike in as many games looked to have ensured bragging rights for the Watford boss but Batshuayi and Costa had other ideas.
Victory gives Conte two wins from two in the Premier League, while Mazzarri is still waiting for his first success in the Watford hotseat.
After early Chelsea pressure, an unchanged Watford side settled into their rhythm on Mazzarri's home debut, and Jose Holebas brought the first save out of Thibaut Courtois after seven minutes.
Watford's attempts to maintain their positive start were nearly undone moments later as Hazard created room for himself on the left and his low cross looked destined for Costa at the back post only for Miguel Britos to make a crucial clearance.
The hosts were holding their own, though, and midway through the half Nordin Amrabat bamboozled Cesar Azpilicueta on the right before delivering a dangerous cross into the six-yard box that Odion Ighalo was unable to convert under pressure from Gary Cahill.
With half-time looming a raking cross-field pass from Branislav Ivanovic was superbly brought down by Nemanja Matic in the Watford penalty area. His cushioned volley saw Costa slide in with Gomes, with the goalkeeper able to claim at the second attempt.
Ten minutes after the restart the game had the goal it had been missing as the hosts struck through Capoue, the midfielder controlling an Adlene Guedioura cross on his chest before lashing past Courtois.
Falling behind for the first time this season fired Chelsea into life and the visitors felt they should have been awarded a penalty 10 minutes later when a left-wing corner appeared to be handled, however referee Jon Moss waved away their appeals.
Their pressure eventually told, though, as Batshuayi - one of three substitutes introduced by Conte - made an immediate impact, the striker in the right place at the right time to tap home from six yards.
And with time running out Chelsea snatched all three points, Costa released by substitute Cesc Fabregas to leave Conte dancing with joy on the sidelines.