Watford stunned FA Cup holders Arsenal at the Emirates Stadium to book their place in the semi-finals at Wembley.
Odion Ighalo scored his first goal in 599 minutes to put the away side ahead early in the second half after Arsenal had spurned a series of good chances.
Adlene Guedioura then smashed home with the visitors’ second shot on target.
Substitute Danny Welbeck scored with two minutes left to set up a dramatic finish, before missing the chance to force a replay from an open goal.
With Costel Pantilimon beaten after an Alex Iwobi effort had come back off the post, Welbeck could only stab the ball agonisingly wide as he struggled to keep his balance on the turn.