Steve Morison sent League One Millwall into the last 16 of the FA Cup as his 85th-minute volley knocked out Premier League Watford.
Morison met Shane Ferguson’s angled cross and shot through the legs of substitute keeper Heurelho Gomes.
It was no more than Millwall deserved – they hit the bar through Lee Gregory after just 17 seconds and forced two fine saves from Gomes.
Watford, semi-finalists last season, managed just one shot on target.
Gomes, meanwhile, kept out volleys from both Gregory and Morison, before the latter landed the decisive blow.
Reaction from all of Sunday’s FA Cup ties
Watford’s fringe players fail audition
Millwall have now beaten two Premier League sides in this season’s FA Cup, and this performance was every bit as impressive as the victory over Bournemouth three weeks ago.
With a league match against Arsenal at Emirates Stadium less than 54 hours after the end of this tie, Hornets boss Mazzarri made seven changes to his team.
Not only did his fringe players fail to impress, he also lost goalkeeper Costel Pantilimon, who was carried off on a stretcher in the first half after challenging Gregory.
The Hornets are still awaiting international clearance for forward M’Baye Niang, signed on loan from AC Milan, while Mauro Zarate is still suspended from his time at Fiorentina.
The sooner they are available the better as far as Mazzarri is concerned, with Watford, 14th in the table, left to focus on securing their top-flight status.
More to follow