John Brayford believes Sunday's FA Cup clash against Nottingham Forest will be the Blades toughest challenge to date, despite United having already won at Aston Villa and Fulham.
Forest arrive at Bramall Lane 16 games unbeaten in all competitions, but Brayford insists the hosts must enter the contest fearless as they have done in previous rounds.
Looking ahead he remarked: "It is going to be a great tie. The lads have done so well to get to this stage and we have to relish this opportunity, especially at home.
"It might not look as difficult as previous rounds did against Premier League opposition but Forest will see this as a great chance to progress to the quarter-final stage themselves and they are certainly in excellent form.
"But there aren't too many opportunities for a League One team to get into the quarter-finals of the FA Cup, we have that chance now and we have to believe we are capable of causing another upset."
The Cardiff City loanee added: "We should be proud of being the lowest ranked team still in the competition. Some people might say it as a negative but I think it is something we should embrace.
"We are here on merit. We have beaten two Premier League clubs and deserve a shot at getting through to the quarter-final."