The hosts have already beaten second-tier opposition to reach this stage, having triumphed away at Coventry City in the last round, while their visitors got past Millwall at the Den.
Match preview
In the midst of a push for promotion to League Two this season, Wrexham began their FA Cup journey in the fourth qualifying round and eventually got past Blyth Spartans in a replay, before dishing out 3-0 and 4-1 thrashings to Oldham Athletic and Farnborough in the first and second rounds respectively.
Such a beating of Farnborough saw the Red Dragons book a third-round trip to Championship side Coventry City earlier in January, and they headed into that tough encounter on the back of four straight victories including a win over Scunthorpe to also punch a ticket in the fourth round of the FA Trophy.
Phil Parkinson's side took a somewhat surprising two-goal lead in their third-round tie through Sam Dalby and Elliot Lee, and after Ben Sheaf pulled one back for Coventry, goals from Thomas O'Connor and Paul Mullin were enough to ensure they eventually came away with a memorable 4-3 victory from a dramatic game and booked another big clash with second-tier opposition.
Although they would then drop out of the FA Trophy, the Welsh outfit do now come into the weekend's test in high spirits having won three further league games to take the run to six straight victories, most recently beating Gateshead 3-0 away from home on Tuesday to move to the summit of the fifth tier as O'Connor, Ollie Palmer and Mullin got on the scoresheet.
Now on a high having leapfrogged Notts County in the fight for a single automatic promotion spot, Parkinson's men will turn their attention back to the FA Cup and look to pull off what would be another major upset to advance to the round of 16.
They meet a Sheffield United side who are enjoying a fine run of form of their own, with the Blades unbeaten in all competitions since their return from the World Cup break in December.
In that time, Paul Heckingbottom's side have played eight games and won seven, including four straight Championship wins to further stretch their lead in the top two before drawing away at Queens Park Rangers and travelling to Millwall in the third round of the FA Cup - their first outing in this year's iteration of the competition.
Daniel Jebbison gave them the lead in the 23rd minute of that contest, and after Jayden Bogle made it two, the Blades stood strong to see out the victory and advance to the fourth round, before continuing their unbeaten run in the Championship with a 3-1 beating of Stoke City thanks to another two goals from Bogle after Iliman Ndiaye's early opener.
They now travel to Wales on the back of their 17th league win of the season last Friday, as Jebbison's early goal made the only difference in a 1-0 triumph at home to Hull City to stretch their lead to 13 points over third-placed Watford in their own fight for automatic promotion to the Premier League.
Now with their sights set on advancing to the final 16, having already given themselves an enviable lead in the race for top-flight football, Heckingbottom's men will aim for a second consecutive away win in this year's FA Cup.
Wrexham FA Cup form:
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Wrexham form (all competitions):
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Sheffield United FA Cup form:
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Sheffield United form (all competitions):
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Team News
Parkinson could be tempted to deploy a very similar Wrexham starting XI to the one that triumphed at Coventry in the previous round, and Paul Mullin is bound to lead the line once again, having already netted 26 goals in 30 appearances in all competitions so far this term and 54 goals in 69 games since arriving at the Racecourse Ground at the beginning of last season.
There is competition to join him up front, though, with Sam Dalby starting and opening the scoring in the third round, while Ollie Palmer started in Tuesday's win over Gateshead and has netted 13 goals so far this campaign.
New signing Andy Cannon has a claim to a starting berth in defence or midfield after arriving recently from Hull City, although Thomas O'Connor and Luke Young are particularly established in the engine room, while Ben Tozer, Jordan Tunnicliffe and Aaron Hayden could again line up in the middle of the back line in front of goalkeeper Mark Howard, who was the first-choice stopper in the Sheffield United side that reached the FA Cup semi-finals in the 2013-14 season.
The Blades should remain without the likes of Ciaran Clark, Enda Stevens, Rhys Norrington-Davies, John Fleck and Rhian Brewster through injuries, and while Oli McBurnie did return in a substitute appearance in the Hull City win, he may not be risked from the outset after a lengthy absence.
Instead, Daniel Jebbison should again lead the line after hitting the net in wins over Millwall and Hull recently, and veteran striker Billy Sharp could join him up front if star man Iliman Ndiaye is afforded a rest.
Oliver Norwood continues to play a key role in midfield, with the likes of Sander Berge, Ben Osborn and Manchester City loanees Tommy Doyle and James McAtee fighting to feature around him, while Chris Basham and Adam Davies could come in in defence and between the sticks respectively if Heckingbottom opts to rotate further.
Wrexham possible starting lineup:
Howard; Cannon, Tozer, Hayden, Tunnicliffe, McFadzean; Lee, Young, O'Connor; Mullin, Palmer
Sheffield United possible starting lineup:
Davies; Basham, Egan, Robinson; Bogle, McAtee, Norwood, Doyle, Lowe; Sharp, Jebbison