United's U23s booked their place in the quarter finals of the Premier League Cup with a dramatic extra-time victory on Tuesday night.
A late goal in normal time from Simeon Oure followed by a strike from substitute David Parkhouse saw the Blades progress to the last eight.
United had the first chance of the game after a cagey opening quarter of an hour as Smith got on the end of a clearance and set up Oure who sliced wide from just inside the box.
On 20 minutes the Blades threatened again when a Semple header sent in Smith but his lobbed effort was blocked by Roos.
The visitors came close themselves shortly after but Graham would clear a dragged effort off the line following a counter attack.
Derby took the lead against the run of play on 28 minutes as a short back pass from Portman was picked up on by WALKER who slotted past Eastwood.
County found themselves down to 10 men just five minutes into the second half as Bogle was dismissed for a dangerous elbow on Semple.
The Blades looked to make immediate use of the man advantage but Semple headed just wide from a Broadbent cross on the hour.
A great opportunity came United's way with the clock on 68 minutes as Cantrill headed back across goal from a Gilmour corner for Semple but his header was saved by Roos.
United deservedly found an equaliser with seven minutes remaining though as a prolonged spell of pressure culminated with Semple chipping the ball to the back post for OURE to bring down and smash home to force the tie into extra time.
The momentum from the end of normal time carried over into extra time for the Blades as they were immediately on the front foot with York's effort being deflected an inch wide of the post.
Derby themselves would come close to going in-front as Babos met a cross from the left and forced an exceptional stop from Eastwood who had to use his feet to thwart the danger.
Chances were flowing at Bracken Moor with Roos being called into action again this time denying Smith one-on-one following a great Slater through ball.
This flurry of opportunities would lead to a goal on 102 minutes with Smith's parried shot falling kindly for PARKHOUSE to score on the rebound to complete the Blades comeback.
Parkhouse came close to doubling his tally in the second period of extra time but his header at the back post from a Slater corner went just wide.
Blades: Eastwood, Cantrill, Portman (Potts 74), York, Graham (Parkhouse 45), Semple, Gilmour, Slater, Smith, Oure (Belehouan 120), Broadbent.
Unused: S. Kelly, Ompreon.