Manchester United booked their place in the FA Cup final after penalty drama and will be back at Wembley in June to take on their noisy neighbours.
The contest couldn't be settled in 90 minutes with both sides enjoying purple patches with the ball, but chances were very much at a premium. The Seagulls enjoyed the better of the play as they eyed a third successive win over United following victories this season and last.
Extra-time followed a familiar pattern with the game becoming increasingly stretched. Marcus Rashford had a deflected effort impressive saved by Robert Sanchez whilst the Seagulls continued to spurn their impressive positions with poor touches and passes.
