90' SAVE! Moments after equalising, the hosts almost go ahead. Martinez comes to Arsenal's rescue on this occasion, the keeper getting low down to his right to deny Sharp.
85' Arsenal have registered just a single shot on target since Pepe's opener - and it come from that same man back in the 29th minute.
82' McBurnie holds his face in his hands after a long ball almost dropped perfectly for him in the box.
81' Saka wins a corner for Arsenal, allowing them to shave a few seconds off the clock. United deal comfortably with the delivery.
80' Arsenal showing reasonable defensive resolve as we head into the final 10 minutes. Just the one goal separates the teams.
78' In comes another United corner and again it's directed at McBurnie. Robinson gets there first, but his header balloons over the bar.
75' Arsenal have struggled to deal with McBurnie's aerial prowess all game and his header from a free-kick almost creates a shooting opportunity, but it's scrambled away.
73' United get numbers forward and await a cross until Tierney makes an excellent challenge to dispossess Kieron Freeman.
70' Around 20 minutes remaining at Bramall Lane and the players have another chance to rehydrate. The home side have threatened in this second half without managing to respond to Pepe's first-half penalty.
69' Tierney, who has shown a good engine in this game, delivers another dangerous ball into the area. It breaks for Xhaka to shoot from distance, but United get bodies in the way.
66' The Gunners need to steady and they do that, momentarily, through some possession in the opposition half. That comes to an end when Lacazette fails to read Willock's flick.
64' Arsenal have gone on to win the FA Cup in three of the six seasons they've knocked out Sheffield United: 1935-36, 2002-03 and 2004-05.
60' ALMOST THE EQUALISER! Basham should score. Robinson sends in an excellent left-sided cross which Basham meets at the far post, but the defender nods wide when he seemed destined to score. Arsenal living very dangerously.
58' OFFSIDE! Wilder's men are foiled once more. Norwood's brilliant free-kick in between Martinez and the Arsenal defence causes chaos as McGoldrick gets a touch before Egan forces the ball over the line. Both men, however, appeared to be offside and a VAR review confirms the assistant's decision.
56' Well, the Blades not all that far from an equaliser and it was almost Henderson who provided it! The keeper's long clearance forward catches the wind, bounces, and almost dips over Martinez. Thankfully for him, it drifts just over the bar.
53' David Luiz appears to be struggling a bit. He's gone down to the ground after shepherding the ball out of play.
51' United remain on the front foot as Fleck drives in a half-shot, half-cross which fizzes harmlessly across the face.
50' Martinez now ventures a long way off his line to push clear a cross from the right. Not entirely convincing but effective enough.
48' ON TARGET! The long throw comes in from the left and McBurnie wins it, but Martinez is positioned well and holds on to the header. Arsenal's defence being asked some questions.
46' Better from the Blades almost immediately as Berge gets a cross into the box. They could do with some thrust from midfield.
46' Back underway at Bramall Lane as Sheffield United trail Arsenal 1-0 with 45 minutes to play.
Arsenal dominated possession during that first half and will look for more of the same to keep their defence out of the spotlight. If the Blades can get more from their front two - perhaps Mousset or Sharp can offer something from the bench - then there is still very much a game to be had.
Arsenal good value for their lead despite receiving something of a let-off in the early stages. It was Lundstram, later substituted with a shoulder injury, who had the ball in the net first, but he was offside when he nodded in McBurnie's header back across goal. Basham's clumsy foul send Pepe to the spot around 15 minutes later and the visitors are now one good half away from an FA Cup semi-final spot.
45' + 7' HALF-TIME: SHEFFIELD UNITED 0-1 ARSENAL
45' + 4' Another head clash, this time between Tierney and Baldock in an aerial duel. Tierney comes off worse but he's fine to play on after being checked.
45' + 2' David Luiz, relatively untroubled at the back to date, gets forward for a set-piece and makes first contact, but he's unable to steer it goalward with Berge holding a handful of his shirt.
45' Action at both ends! Arsenal keeper Martinez first holds a curling Norwood effort. The Gunners then get at their opposition with Tierney able to overlap on the left and spot Pepe's run, which he rewards with a fine cut-back, but the Ivory Coast international finishes the wrong side of the upright.
43' Arsenal threaten again! Maitland-Niles gets around the United defence on the right and draws an involvement out of Henderson from an attempted cross. The ball eventually comes out to Tierney, who shoots past the left post.
41' Good omen for Arsenal? Pepe's opener was their first goal from the penalty spot in the FA Cup since Arteta netted against Everton in 2013-14, also at the quarter-final stage. The Gunners went on to win the competition that season.
39' Both players now back to their feet and will seemingly continue.
37' Concern now for McBurnie and Willock after an ugly, accidental clash of heads off the ball. No malicious intent. Both players down receiving treatment.
34' Quick, crisp Arsenal passing leads to a crossing opportunity for Tierney. They're moving the ball well through midfield. Meantime, Lundstram is down receiving some treatment on his left shoulder after tangling with Willock.
32' Arsenal win another corner, which McBurnie heads clear. Arteta's men are asserting their dominance now.
29' SAVE!! And it's a very, very sharp one from Henderson, who gets down to his left to push Pepe's stinging shot around the post. Good play all round.
29' United win a free-kick in a central area and Fleck proceeds to lash it low into the wall.
27' That goal is the final action before a brief break for the players. The penalty came after Tierney passed into Lacazette's feet, inviting the challenge from Basham.
23' PENALTY!! Basham clips Lacazette's ankle just inside the box and Arsenal have a spot-kick. Huge opportunity here for the visitors.
22' Pepe reaches the byline, swings in a right-footed cross and forces Robinson to clear for a corner.
20' Willock misjudges the flight of the ball and a promising Arsenal move comes to nothing.
19' United last played in an FA Cup quarter-final in 2013-14 as a League One side, winning 2-0 against Charlton Athletic. They ultimately lost out against Hull City in the final four.
17' One point separates these teams in the Premier League and a close contest seems set to unfold over the next 75 minutes or so.
15' Arsenal breathing a sigh of relief after those two close calls. They were outdone from the corner that Lundstram nodded home and are perhaps fortunate not to be behind.
13' CHANCE! United go close again. Robinson's long throw is flicked on at the near post and only marginally eludes McGoldrick. All the striker needed was a touch.
10' DISALLOWED! The Blades look to take the lead as McBurnie gets his head to a corner, sending it back across goal for Lundstram to nod home. But replays show Lundstram was offside and the goal is chalked off following a VAR review. Big let-off for Arsenal!
7' SHOT! The first attempt of the match comes in from the centre-back Robinson, whose deflected effort goes out for a corner. Good, adventurous move from the hosts.
7' The Gunners build well out of defence and move the ball into midfield before Saka wins a foul.
5' It appears as though Tierney will fill the wing-back role on Arsenal's left flank today, with Kolasinac behind him in the three-man defence.
3' An early free-kick in a decent area for the Blades but Norwood's delivery goes beyond everyone and out for a goal-kick.
2' It will be interesting to see how quickly Arsenal settle after making five changes. The club has such strong FA Cup pedigree but this team has much to prove.
1' KICK-OFF! We're underway at Bramall Lane. It's Sheffield United and Arsenal battling for a place in the FA Cup semi-finals.
The teams are out on the pitch and it's almost time to get started.
Arsenal have won just one of their past eight away matches against Sheffield United in all competitions, drawing four and losing the other three. The sole victory was a 3-0 success in the EFL Cup back in October 2007.
Eight changes in all between the two teams. There are five for Arsenal, who bring David Luiz straight back in from suspension and add Maitland-Niles, Willock, Kolasinac and Lacazette to their starting XI. Mesut Ozil misses the matchday squad due to a "slight back injury," according to the club. Egan and McBurnie return for the Blades, as does goalkeeper Henderson after a clause in his loan deal forced him to miss the midweek defeat to parent club Manchester United.
ARSENAL SUBS: Matt Macey, Hector Bellerin, Sokratis Papastathopoulos, Rob Holding, Matt Smith, Dani Ceballos, Reiss Nelson, Eddie Nketiah, Pierre-Emerick Aubameyang.
ARSENAL XI (3-4-3): Emiliano Martinez; Shkodran Mustafi, David Luiz, Sead Kolasinac; Ainsley Maitland-Niles, Granit Xhaka, Joe Willock, Kieran Tierney; Nicolas Pepe, Alexandre Lacazette, Bukayo Saka.
SHEFFIELD UNITED SUBS: Simon Moore, Phil Jagielka, Kieron Freeman, Luke Freeman, Ben Osborn, Sander Berge, Richairo Zivkovic, Lys Mousset, Billy Sharp.
SHEFFIELD UNITED XI (3-5-2): Dean Henderson; Chris Basham, John Egan, Jack Robinson; George Baldock, John Lundstram, Oliver Norwood, John Fleck, Enda Stevens; David McGoldrick, Oli McBurnie.
Neither side has looked particularly assured since returning to action earlier this month. The Blades have been especially disappointing, recording successive 3-0 losses following their goalless draw against Aston Villa. It's otherwise been an exceptional campaign for Chris Wilder's men and an FA Cup semi-final appearance at Wembley would be a fitting reward for the progress made prior to the season going on hold.
Mikel Arteta finally registered his first Premier League away win as Arsenal head coach with a 2-0 victory over Southampton on Thursday. Now comes a trickier challenge. The Gunners are battling injury problems, remain on the road and must overcome an opponent with nothing to lose if a record-extending 14th FA Cup crown is to remain within reach.
Hello and welcome to our live coverage of this FA Cup quarter-final clash between Sheffield United and Arsenal!