John Egan's stoppage-time goal rescued Sheffield United against non-league Wrexham in a frenetic FA Cup fourth-round tie.
Oli McBurnie headed home Tommy Doyle's corner to give United an early lead.
Goals from James Jones and Tom O'Connor put the Welsh side ahead before Oliver Norwood equalised for the Blades, who had Daniel Jebbison sent off.
Paul Mullin's goal four minutes from time looked to have secured a shock, but captain Egan levelled late.
Egan's injury-time strike proved to be the final twist of an enthralling game which pitted the National League leaders against the team second in the Championship.
Wrexham co-owner, Hollywood star Ryan Reynolds, was at the Racecourse to see his side put in a tremendous display in their first fourth-round appearance since 1997.
Naming an unchanged team, Dragons boss Phil Parkinson said the aim was to make it an afternoon as "difficult as possible" for a visiting side pushing for a place in the Premier League.
Parkinson accepted Wrexham, unbeaten in 16 league games, went into the game as underdogs - just as they did prior to the previous round's win at Championship outfit Coventry.
Sheffield United showed five changes to the side which beat Hull City 1-0 to extend their unbeaten run to nine games.
And from the kick-off the Blades showed their intent, winning a corner inside the opening two minutes from which the returning McBurnie headed home.
Wrexham's hopes suffered a double blow with defenders Jordan Tunnicliffe and Aaron Hayden forced to leave the field with injuries with barely seven minutes of the game gone.
But the hosts responded positively to those setbacks, with Mullin leading the charge as he looked to add to the seven goals already netted in this season's competition.
The visitors were looking dangerous on the break and a strong run and subsequent shot from Jebbison was palmed away by Mark Howard, with the former Blades keeper also denying Tommy Doyle soon after.
Both sides played their part in an entertaining, end-to-end cup tie, which did not let up at any stage.
Elliot Lee had a shot from the edge of the area blocked before Mullin was brought down on the edge of the penalty box by Egan, but he struck the free-kick against the United wall.
Having ended the first half strongly, Wrexham made a storming start to the second period.
Captain Luke Young's shot saved was by Adam Davies with Mullin failing to finish with the rebound before the United defence cleared.
But the home side did level in the 50th minute when a trademark Ben Tozer long throw was hooked on by O'Connor and substitute Jones pounced.
Wrexham were on top after that and when the United defence failed to clear a 61st-minute corner, O'Connor smashed the ball home to send the home supporters into raptures.
Celebrations had barely died down when Paul Heckingbottom's men equalised against the run of play four minutes later.
Wrexham failed to clear substitute Billy Sharp's cross and Norwood coolly slotted home past Howard after a first effort was blocked.
The terrific and tense tie took another turn when Jebbison was shown a red card in the 71st minute following an off-the-ball clash with Tozer, in which he kicked out at the defender.
Wrexham pressed for a winner and with four minutes of the 90 remaining, Mullin collected a pass from substitute Sam Dalby before placing the ball under Davies for his 27th goal of the season.
But their hopes of adding another scalp to their notable FA Cup history were dashed in stoppage time, with Egan finding the net from Doyle's corner to ensure the sides will meet again at Bramall Lane to decide who goes through to the fifth round.
Wrexham manager Phil Parkinson said:
"The start we had, conceding a goal and losing two players, for the team to reset ourselves as well as we did I thought was outstanding.
"A lesser group of players could have got beaten four or five-nil there. We had to rejig the team around and the way we regained our composure was absolutely outstanding.
"We got to half-time and I believed at half-time we would get back in the game. We took the game to Sheffield United, pinned them in for long periods.
"Obviously it's a real blow that we haven't got over the line, so it's mixed emotions as you can imagine.
"But the way the lads performed on the big stage was outstanding."
Sheffield United manager Paul Heckingbottom said:
"We knew how difficult it was going to be - they've won every home game - so we knew how tough it was going to be, so no surprises whatsoever.
"We all know why everyone turned up here today, I'm just glad we've not given everyone what they wanted.
"That was our motivation so we're pleased to be in the fifth-round draw."
Line-ups
Wrexham
Formation 5-3-2
21Howard
14Forde5Hayden4Tozer6Tunnicliffe3McFadzean
8Young22O'Connor38Lee
10Mullin9Palmer
21Howard14Forde5HaydenSubstituted forJonesat 11'minutes4TozerBooked at 71mins6TunnicliffeSubstituted forCleworthat 5'minutes3McFadzeanBooked at 64minsSubstituted forDalbyat 75'minutes8YoungBooked at 56mins22O'Connor38Lee10Mullin9PalmerSubstitutes
1Lainton2Hall-Johnson11McAlinden18Dalby20Cannon26Lennon27Bickerstaff30Jones32CleworthSheff Utd
Formation 3-5-2
1Davies
15Ahmedhodzic12Egan6Basham
20Bogle28McAtee16Norwood22Doyle23Osborn
36Jebbison9McBurnie
1Davies15AhmedhodzicSubstituted forCoulibalyat 87'minutes12EganBooked at 45mins6Basham20BogleBooked at 90mins28McAteeSubstituted forLoweat 45'minutes16Norwood22Doyle23OsbornSubstituted forNdiayeat 58'minutes36JebbisonBooked at 71mins9McBurnieSubstituted forSharpat 58'minutesSubstitutes
10Sharp13Lowe17Coulibaly18Foderingham19Robinson29Ndiaye39Sachdev41MarshReferee:Dean WhitestoneAttendance:9,949Live Text
Match ends, Wrexham 3, Sheffield United 3.
Second Half ends, Wrexham 3, Sheffield United 3.
Goal! Wrexham 3, Sheffield United 3. John Egan (Sheffield United) right footed shot from the centre of the box to the bottom left corner. Assisted by Tommy Doyle with a cross following a corner.
Corner, Sheffield United. Conceded by Thomas O'Connor.
Attempt missed. Iliman Ndiaye (Sheffield United) header from the centre of the box is close, but misses to the left. Assisted by Tommy Doyle with a cross.
Jayden Bogle (Sheffield United) is shown the yellow card for a bad foul.
Elliot Lee (Wrexham) wins a free kick in the attacking half.
Foul by Jayden Bogle (Sheffield United).
Attempt missed. Sam Dalby (Wrexham) left footed shot from the centre of the box is high and wide to the left.
Substitution, Sheffield United. Ismaila Coulibaly replaces Anel Ahmedhodzic.
Goal! Wrexham 3, Sheffield United 2. Paul Mullin (Wrexham) right footed shot from the centre of the box to the centre of the goal. Assisted by Sam Dalby following a fast break.
Attempt saved. Billy Sharp (Sheffield United) right footed shot from a difficult angle and long range on the left is saved in the top centre of the goal. Assisted by Oliver Norwood.
Foul by Paul Mullin (Wrexham).
Chris Basham (Sheffield United) wins a free kick in the defensive half.
Attempt blocked. Elliot Lee (Wrexham) right footed shot from outside the box is blocked.
Ollie Palmer (Wrexham) hits the bar with a right footed shot from the centre of the box.
Luke Young (Wrexham) wins a free kick in the defensive half.
Foul by Jayden Bogle (Sheffield United).
Attempt missed. Thomas O'Connor (Wrexham) header from the centre of the box is too high. Assisted by Ben Tozer.
Substitution, Wrexham. Sam Dalby replaces Callum McFadzean.
