Shootout heroics from Chris Maxwell and ice cold composure from Nick Haughton were crucial as Fleetwood Town edged closer to Wembley.
It was by no means an easy victory, Town once again struggling for goals, forced to endure the agony of their first ever shootout.
But two spot kick saves from Maxwell decided the tie, Haughton applying the final nail to the Blades' coffin.
With 13 changes between them the two sides could be forgiven for a touch of uncertaintly in the opening exchanges.
The Blades were the side with the majority of new faces but it was Fleetwood who looked the more shaky, a couple of indecisive moments almost allowing United to grab the initiative.
Town conceded within two minutes when the sides met at Bramall Lane last months.
They avoided a similar fate this time around, but had Chris Maxwell to thank for reaching the 10 minute mark with the scores level.
Fleetwood were twice found wanting at the back - Matty Done the first to stretch the hosts, charging in between the centre half pair of Nathan Pond and Dionotan Teixeira, sliding the ball to Martyn Woolford whose blast from 12 yards was parried by the Fleetwood keeper.
The next test came down the Fleetwood left, Woolford this time the creator, hooking the ball inside for Conor Sammon whose shot was pushed up in the air by Maxwell, caught before any more chaos could ensue.
Town, having kept the bodged Blades at bay, began to create openings of their own, moving the ball neatly and quickly down both flanks.
But the troubles in front of goal which had plagued them late on at the Bescot on Saturday showed no sign of easing, George Long perhaps a little fortunate not to have had to pick the ball from his goal.
Jamille Matt was so nearly the architect of what would have been an excellent opener, a neat touch setting him up just beyond the 18 yard line, Tyler Forbes making the run into the area, closed down a he pulled the trigger.
Declan McManus was poised for the follow up, letting fly a stinging effort from six yards, Long only able to turn the ball behind.
Fleetwood's purple patch continued, Eggert Jonsson finding room to stretch his legs down the left, Matt fluffing his lines with the cross from the channel.
It fell to Teixeira who took one touch, two, three, four... too many, a shot eventually dispatched but United untroubled.
All of it was typical of Fleetwood's play right now, at times providing the creativity and penetration but all too often lacking the end product to make it count.
And as quickly as the flurry of goal mouth activity had begun, it faded away, the game transformed into a midfield stalemate, a contest of pressure, nerves and never ending minor errors.
Both sides were guilty, Fleetwood with careless passing, the Blades let down by an number of heavy touches.
There were half chances, a deflected Woodford shot flashing past the upright, a poorly defended set piece almost opening the door for Done before the offside flag was raised.
But the early promise of end to end football had evaporated.
Fleetwood, on the rare moments they did hook something together proved what a side they might be with a touch more pace.
Amari'i Bell deserved recognition for his attacking contribution from left back, almost forcing the issue before the break.
He showed a decent touch and good vision to surge forward, curling a pass across the face of goal.
In came Forbes, the winger almost connecting, the Blades forced to concede the corner as they were, for a moment, rattled.
But Nigel Adkins men soon found their feet again, carving Town open down the middle once more, Done looking to have Maxwell beaten with his final ball until Nathan Pond slid in to ensure the scores were level at the break.
The hosts made a lively start to the second half, Forbes continuing to make good use of his quick feet in wide areas.
When he skipped past Kieron Wallace Town looked to finally have the opening the needed, the cross not matching the clever work on the ball.
But Town weren't to be put off, the ball falling back to Forbes who curled a ball across the face, beating Long, McManus just losing the race as the ball drifted harmlessly past the back post.
The game seemed, once again, to have fallen into an attacking groove, Terry Kennedy firing past the upright from a Blades corner, Forbes then making ground to trouble the Blades down the right, his second bite of the cherry nearly creeping in at the back stick with McManus two steps away from applying the final, decisive touch.
Stefan Scougall went closer still, stinging the palms of Maxwell from 18 yards as the action flowed from end to end.
It looked like fresh legs might be the boost Fleetwood needed, David Ball replacing Jamille Matt. But instead the fizz went out of the game - Town still enjoying plenty of possession but the sides once more bogged down in a frustrating game of cat and mouse.
In truth, box to box they were unquestionably in control.
But the failure to attack half opportunities, to grab the game by the scruff was keeping the tie open.
Town were agonisingly close, Nick Haughton almost releasing Ball, a last ditch tackle from Kieron Wallace saving the Blades.
From the corner sub Ash Hunter let fly from 20 yards, forcing Long to push his shot around the post as Town showed they have the quality to compete.
Fleetwood looked the most likely to settle it, Bobby Grant looking determined to have his say having come off the bench.
He bludgeoned his way into the area, leaving stumbling defenders in his wake, going for glory but unable to beat Long.
Hunter did find a way past, a 25 yard stunner looking certain to settle the tie, the woodwork denying Fleetwood as the shot hit the underside of the bar and spun out - Long getting the slightest of touches as the ball spun over his head.
It would have been a goal worthy of winning any cup tie - Fleetwood's luck simply not in.
To penalties it went, Bobby Grant giving Fleetwood the perfect start, Jose Baxter responding.
David Ball put the hosts back ahead, Maxwell diving to deny Billy Sharp.
Jimmy Ryan kept the hosts on track, Maxwell then repeating his heroics, at full stretch to deny Jay McEveley.
Young midfielder Nick Haughton made no mistake, firing low to Long's left to secure progress for Fleetwood.