Salomon Kalou should have given the Premier League side a first-minute advantage but failed to commit enough to Didier Drogba's ball across the six-yard area, before Drogba, himself, stung Watford keeper Scott Loach's hands with a swerving shot from 25 yards.
The Blues' passing in those opening moments was sublime and the gulf in class could not have been more exposed, but the Hornets defence, brilliantly marshalled by Jay DeMerit, settled down and focused on blocking Chelsea's supply line which, unimaginatively, was largely coming through the centre.
Frustration began creeping into the visitors' play, who were put together for the match by assistant Ray Wilkins, and Loach's only other call to action of the half came when he had to punch away Lampard's dipping free-kick.
However, the 20-year-old shot-stopper was beaten for the first time after a one-two between Anelka and Drogba finished with Frenchman Anelka placing his low shot past Loach but on to the post.
Watford did not offer too much as an attacking force in the first 45 minutes, although Cech did have to push away Grzegorz Rasiak's far-post header from a long cross-field ball.
Brendan Rodgers' side offered even less after the interval as Chelsea turned up the heat and romped forward with Drogba testing Loach again with a low drive before seeing his goalbound drive blocked by his team-mate Ashley Cole.
So to say that Watford's goal came against the run of play would be a massive understatement.
Priskin, who had barely been on long enough to get his boots muddy, ran on to a long pass, sprung the offside trap and sent his shot in off Cech and into the empty net.
Another shock for Chelsea following their league cup exit to Burnley? It was on the cards for just six minutes before Anelka's quick-fire double turned the game around.
The first came from a Lampard corner, which was flicked on by Michael Ballack before the striker's overhead flick ended up high in Loach's goal.
Cole then found Anelka with a left-wing cross who headed his effort down and past the Watford keeper.
The comeback was complete in injury time when the French striker picked up Kalou's pass, turned and fired in a deserved third.
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