If City were regarded as underdogs, they were clearly having none of it and Steve Cotterill’s team tore into their Championship opponents from the first whistle.
Watford were taken aback by City’s high-tempo start and the visitors ought to have gone ahead after just three minutes, Sam Baldock finding himself one-on-one with Jonathan Bond, only to shoot straight at the goalkeeper.
When the home side finally settled, influential midfield play-maker Lewis McGugan tried his luck from range, drawing a routine save from Elliott Parish.
But it was League One City who continued to create chances of the clear-cut variety and Baldock headed over after Aden Flint had nodded Marlon Pack’s cross into his path, while Joe Bryan forced Bond to punch clear at his near post.
Beppe Sannino’s side carried a threat on the counter-attack and Flint pulled off a brilliant last-ditch tackle to deny the recalled Fernando Forestieri a clear sight of goal, while Ikeche Anya’s powerful shot was blocked as the Robins defended in depth.
Watford pressure eventually told in the 25th minute, Ikeche Anya attacking down the left and forcing a corner. McGugan whipped the ball in towards the far post and Davide Faraoni’s shot went in off Karleigh Osborne.
Worryingly for City, it was an identical goal to the one they conceded at Bradford just four days earlier.
The visitors tried to respond and Marvin Elliott narrowly failed to connect with a Brendan Moloney cross, but there was little to seriously trouble a Watford defence that appeared comfortable as the wind went out of City sails.
When Jay Emmanuel-Thomas finally engineered room to unleash a shot, his curling effort from 18 yards was safely gathered in by Bond.
City finished the first half in the ascendancy and Bobby Reid, at full-stretch to reach Elliott’s centre, fired high and wide from eight yards out.