London - Third tier Tranmere knocked out last season's FA Cup finalists Watford on Thursday with a 2-1 win after extra time in a third round replay to set up a clash with Manchester United.
The first match between the clubs finished 3-3 after Tranmere recovered from a 3-0 deficit, and last week's scheduled replay was pushed back after heavy rain made the pitch unplayable.
Watford manager Nigel Pearson made 11 changes to the team beaten 2-1 by Aston Villa in the Premier League on Tuesday, prioritising the club's top flight survival over the cup.
Tranmere are struggling in the League One relegation zone but grabbed the lead in the first half at Prenton Park when Emmanuel Monthe volleyed home at a corner.
Watford, who lost 6-0 to Manchester City in last year's final, equalised on 68 minutes through substitute Kaylen Hinds after he drilled through a crowd of players.
Paul Mullin headed in the winner on 104 minutes as Tranmere set up a fourth round tie at home to 12-time champions United on Sunday.
Watford introduced Henry Wise, the son of former Chelsea midfielder Dennis Wise, from the bench for the closing minutes, but he was unable to make a decisive impact.