For Brighton fans it was a second Championship scalp. For the manager, Gus Poyet, it was proof that his League One promotion contenders are ready to play in the division above.
Ashley Barnes scored the winner, but it was thanks to a large slice of clumsiness from the home goalkeeper Rene Gilmartin that Watford's interest in the competition ended yesterday.
Poyet admits promotion to the Championship is the priority, but this, which followed a victory in the previous round over Portsmouth, helped show the Uruguayan that his side do not have far to go to compete with the second tier.
He said: "That's what I used this game for – to test the side. We have a lot of players finishing their contracts this year and it is very difficult to see whether you are going to make it in the Championship or not with these players.
"It proved to me now we have plenty of players who can play in the Championship – and play the way I want, which was the main thing."
The Brighton manager knows what a cup run can do to a side's form, having watched Leeds' League One campaign stutter last year following their third-round victory over Manchester United and a draw against Tottenham Hotspur in the fourth.
But he said: "If it was a lower division team at home I would have played a totally different team. I saw what happened to Leeds last season, they were beating Premier League teams in the FA Cup and losing every game in the League. I didn't want that. We are not doing that, so I am pleased, but I hope it doesn't change. The priority is going up."
Barnes will go on record as scoring the winner, but Gilmartin may as well godown with an assist, after his Robert Green moment let the ball slip through his handsinto the net.
Chris Wood, outstanding for Poyet's side, could have had two himself but for the crossbar and Lloyd Doyley's last-ditch tackle.
The Watford manager Malky Mackay refused to blame Gilmartin, but admitted his men had an off day.
He was right. His side are usually as good at passing football as Brighton were yesterday – and rarely shy of a goal themselves – but it took until the 55th minute before they registered a shot on target, through Martin Taylor's weak effort.
Mackay said: "It was a below-par performance. We didn't test Brighton as well as we should have. The fact that we gave them a goal – and we had two or three chances to protect that move before it got to the goalkeeper – summed it up really.
"Rene has been a good figure for us and in training and in games his standards have been so high, so it would be remiss of me to blame him for today. Some days you have off days and today was one of those."
Referee: Eddie Ilderton
Man of the match: Wood