Nigel Clough's Blades kicked off 2015 with yet another memorable FA Cup victory.
This time it was Harry Redknapp's Queens Park Rangers who felt the force of the Blades.
Marc McNulty gave the Blades a deserved lead and Jamal Campbell-Ryce scored twice in the second half to continue the growing reputation as cup specialists.
Clough made five changes in total, many of which were enforced, allowing Kennedy, Reed, Flynn, Baxter and McNulty to all start, and the Blades more than matched the Premiership side in the opening 30 minutes, creating as many, if not more opportunities.
Referee Clattenburg was unmoved as Hoilett tipped over in the box early on and the first Blades chance saw Baxter pick out McNulty, who slipped at the vital moment. Howard turned over a shot from Fer on the turn following a corner and then the Blades goalkeeper also was ideally placed to save from Mutch after he robbed Scougall.
A couple of Campbell-Ryce crosses put QPR under pressure, including one on 24 minutes from a Harris tackle and pass but McNulty was crowded out at the near post.
From a Phillips free-kick, a Traore shot was blocked before United clearly got on top. Baxter cut inside and shot at McCarthy and McNulty took the attentions of around four home defenders before, moments later, the Scot gave the Blades the lead.
United's midfield runners were causing QPR problems and it was a main reason for the opening goal on 36 minutes. Predominately it was Scougall but on this occasion it was Reed, whose pass beat Ferdinand and it was ideal for McNULTY to slide the ball through McCarthy and into the net.
For the remainder of the half United coolly kept the Premiership side at bay and the only other chance of note was from the Blades but Scougall could not get enough connection on a Campbell-Ryce cross.
At the start of the second half QPR introduced Zamora and the first opportunity after the interval resulted in another goal...for the Blades. An unnecessary corner saw Reed deliver, McCarthy flap, the ball hit Mutch, possibly on the hand, but CAMPBELL-RYCE did not wait for the referee and slotted home from close range - right in front of the travelling contingent.
Predictably, QPR hit back with a number of chances. Zamora's flicked header was wide, Fer's free-kick was blocked and Austin's shot was comfortable for Howard, who made a tremendous reaction save with his foot from Onuoha's header.
On the hour QPR used up their substitutes and changed formation. They continued to dominate but the Blades continued to repel. Howard pushed out from Vargas, Isla's effort was blocked and Phillips, once a Blades loanee, fired over on 73 minutes.
McNulty and Reed picked up tactical yellow cards and United's first change arrived on 82 minutes when Murphy took over from the tiring McNulty. Late on Phillips was again off target and Baxter tried his luck before being replaced by Wallace.
In four minutes of added time, Higdon replaced Doyle but United comfortably saw out the remaining minutes to secure another top-flight scalp and a fourth round place...until 15 seconds remaining when CAMPBELL-RYCE capitalised on a defensive error and picked his moment before slotting home.
QPR: McCarthy, Traore, Caulker (Isla 60), Ferdinand, Phillips, Austin, Fer, Onuoha, Mutch, Henry (Vargas 60), Hoilett (Zamora 45).
Unused: Murphy, Hill, Furlong, Grego-Cox.
Blades: Howard, Harris, Flynn, Doyle (Higdon 90), Scougall, Baxter (Wallace 90), McNulty (Murphy 82), McEveley, Campbell-Ryce, Reed, Kennedy.
Unused: Alcock, McGahey, Turner, Dimaio.