The Blades and Stevenage go into Monday's semi-final second leg on level terms after a goal-less but pulsating tie at the Lamex Stadium.
Danny Wilson's side combatted the physical threat of Gary Smith's side, who went into the game on the back of a six-match unbeaten run.
But the closest the home side came was a Craig Reid effort, which struck the crossbar, which makes Monday's return at Bramall Lane intriguing.
Boss Wilson announced three changes before kick-off with Hill, Quinn and Porter all recalled. Cresswell and Taylor dropped to the bench and Beattie suspended. The new personnel suggested a change in formation and moments after the start it was evident that United were going with a 4-4-1-1 system with Quinn the advanced midfielder, supporting lone striker, Porter.
The opening 15 minutes, in a slow start, saw both sides limited to just a decent cross each. For the Blades, Lowton's delivery was destined for Porter but he was muscled out of it by Stevenage's central defenders, whilst Freeman's free-kick was won by Agyemang but Doyle cleared ahead of Reid.
The cagey start by both teams, keen not to hand over the initiative to the other, continued. Bostwick powered forward but curled his shot over and Freeman hammered across goal, whilst Lowton got a toe in to stop Reid's momentum as Boro looked to use their home advantage.
Freeman's shot was blocked by Doyle and the best opportunity of the half also fell to the home side, on 42 minutes. Freeman nicked the ball from McDonald and laid off to Reid, who curled his effort against the crossbar with Simonsen adding the merest of touches.
The chances appeared to act as a wake-up call for United, who then put pressure on the Stevenage goal before the interval. Quinn let fly with two shots, the first blocked and the rebound sailing over, whilst goalkeeper Day was forced to tip over a header, which appered to come from defender Roberts. From the resulting corner, McDonald's shot from the edge of the box was a yard wide.
United made the brighter start in the second half and a Quinn free-kick needed defending, whilst he also delivered a cross from which Flynn challenged in the six-yard box before Day collected. It was from another Quinn cross, on 57 minutes, that the Blades went close yet again. Porter challenged with Laird at the back post and as the ball dropped, Williamson went down in a heap after a collision with Freeman but referee Bates did not think the visitors deserved a penalty.
Quinn was heavily involved in United's encouraging second half opening and around the hour, following a corner, the Irishman attempted an overhead kick which was cleared by Wilson. From another corner, moments later, Williamson saw his shot saved by Day.
On 70 minutes, from a Quinn pass, Flynn got behind the Stevenage defence and opted not to shoot from a tight angle but his pass inside failed to find either Porter or Williamson, whilst McDonald's shot from the right took at least two deflections before dropping into the arms of the home goalkeeper.
Stevenage's first serious threat on the Blades goal in the second period came from a Freeman corner but Ashton was unable to get a notable connection, despite positioning himself inside the six-yard box. Bostwick fired wide from distance soon after.
As the game entered the final ten minutes, Stevenage used their third and final substitution, whilst United were seemingly content to take a draw as the first leg neared a conclusion, although, on the break, Ashton was forced to keep Quinn's cross away from Porter.
Wilson made a double change as the clock ticked around to 90 minutes, whilst four additional was announced by the fourth official. Late on Roberts prevented Porter connecting with Quinn's cross, whilst Laird cleared a header from Lowton. The final moments were frantic but neither side was able to break the deadlock ahead of Monday's conclusion.
Stevenage: Day, Wilson, Laird, Ashton, Reid (Mousinho 65), Byrom (Shroot 71), Roberts, Freeman, Bostwick, Agyemang, Lascelles (Myrie-Williams 82).
Unused: Julian, Beardsley.
Blades: Simonsen, Lowton, Doyle, Flynn (Cresswell 90), Williamson, Collins, Maguire, Porter, McDonald (Ertl 90), Quinn, Hill.
Unused: Howard, Taylor, O'Halloran.