City recovered from the blow of conceding late at Ashton Gate to earn a replay with last season’s Championship play-off finalists Watford.
The Robins looked destined to bow out of The FA Cup with Budweiser when Sean Murray’s low shot sneaked into the bottom corner from 25 yards with only six minutes left.
But the underdogs fought back immediately, as substitute Wes Burns raced to the byeline straight from kick-off to pick out Jay Emmanuel-Thomas for a dramatic equaliser.
Steve Cotterill made three changes to his side victorious over Stevenage in their last outing of 2013, all enforced.
The most notable of which saw Liam Fontaine return to the starting line-up for the first time since August following a loan spell at Yeovil Town.
The defender replaced the suspended Derrick Williams in a like for like swap, while Greg Cunningham and Simon Gillett also missed out through illness and ineligibility respectively.
In came Joe Bryan and Marlon Pack along with Fontaine to plug the gaps.
It was the visitors who made the more promising start, with former Nottingham Forest midfielder Lewis McGugan driving into the box only to slice a left-footed shot off target at the near post.
City responded through Karleigh Osborne’s hanging cross which had Manuel Almunia flapping underneath it only to be bailed out by the award of a free kick for a nudge by Sam Baldock.
From then on the Robins took the game to Watford. Baldock drilled a long-range shot which Almunia smothered, while Bobby Reid recovered from an early knock to fire one just wide on his left foot.
Then a typical burst from Emmanuel-Thomas carried him into the penalty area but resulted in a less recognisable dragged shot, albeit from a difficult angle.
At the other end, McGugan proved to be the threat again with a solo run which saw him dribble the ball from the left side of the pitch diagonally beyond Aden Flint only to lose balance as he lifted his shot over the bar.
City’s best chance would follow soon after. Scott Wagstaff’s cross from the byeline was taken off the head of Marvin Elliott by Baldock, who inadvertently helped it on and into the path of Bryan at the back post, who had time to take aim and force a crucial save from Almunia.
Elliott lifted his shot from the resulting corner over the bar, before McGugan tested Parish for the first time with a brilliant curling shot expertly turned aside by Elliott Parish.
It was a lively spectacle despite the lack of goals, and Bryan got around the outside to produce a perfectly clipped cross towards the back post which Emmanuel-Thomas attacked in the air but could not divert on target from close range.
It was another City opening but you could still sense Watford growing in confidence. Diego Fabbrini’s close control took him into the box to set up a familiar source, but Fontaine raced out to make a sliding block as McGugan pulled the trigger.
Cristian Battocchio’s cross from the left was met by defender Gabriele Angella soon after, who threw himself at the ball under pressure from Fontaine only to see his diving header clip the top of the crossbar on its way into the Atyeo Stand.
City could be forgiven for feeling a little hard done by to still be on level terms at the interval, but Emmanuel-Thomas set about breaking the deadlock at the start of the second period with a fine threaded pass between the Watford centre-backs for Baldock to chase down with a low left-footed shot well held by Almunia.
It was the only notable incident of the game’s third quarter, but Cotterill’s gamble to bring on Burns for Wagstaff sparked a more open final 20 minutes.
First Watford seized on a misplaced pass in midfield to launch a counter-attack which culminated in loud appeals for a penalty when McGugan went to shoot only to become embroiled in a tangle of legs with Joe Bryan.
The Hornets’ feeling of adversity only prompted another yellow charge, as Deeney was put through but fired straight at Parish.
City’s best chance for some time arrived when Elliott did well to keep Reid’s deep free kick in play, helping the ball back across goal before Osborne headed over with Almunia out of position.
It was a big chance, but the defender was stretching for a ball inches too high for him while Baldock waited to pounce behind his back.
Fabbrini negotiated space for a cross as the game entered its final ten minutes, prompting Burns to head his own goal but uncomfortably close to Parish’s right-hand post.
But what looked like being the killer blow was only just around the corner, as Murray’s unconvincing low shot from distance crept through a crowd of bodies and beyond the unsighted Parish.
At that point Watford looked assured of a place in round four, but Burns’ remarkable burst of pace saw him breeze past Battocchio and find the perfect cutback for Emmanuel-Thomas to convert goal number 18 of a stunning debut season.
There was almost one last twist when Elliott met Emmanuel-Thomas’ stoppage time free kick, but his header lacked the necessary power to trouble Almunia, who gladly held on.