The Tigers play the League One side at Wembley Stadium on Sunday with the winner facing either Arsenal or Wigan Athletic.
Despite Hull's status as favourites going into the match, Bruce is well aware that an upset will be possible if his team does not perform.
"You look at what Sheffield United have done and nothing's a given at Wembley," he told the Hull Daily Mail. "A semi-final is a semi-final. There's nerves, edginess, you need a bit of luck and the bounce of the ball.
"We may never get a better chance of getting to an FA Cup final, but Sheffield United will be thinking exactly the same thing.
"We've got to perform on the day and that's the key. No matter what division you're in, you have to perform on the day."
Hull made it into the semi-finals of the competition with a 3-0 win over Sunderland in the last round.