A late strike from Frank Maguire was the different between a draw and a win for United's Under 18s on a morning where they largely dominated against Crewe Alexandra at the Steelphalt Academy.
United did everything but score during a solid 90 minutes for a young side but it was Maguire who did the damage two minutes into stoppage time.
The Blades went at their opponents right from the off but couldn't find the net or overly trouble the Crewe goalkeeper.
Harrison Neal saw an effort deflected on what became the story of the first half, with Zak Brunt also seeing an attempt on target deflect just wide before Clark Greaves saw his goal-bound header blocked.
United did have one scare during the half when goalkeeper Sam Kelly saw his kick out blocked, only to pick himself up and recover to make a big save from Crewe striker Malachi Linton.
The pressure continued into the second half. Leo Gaxha felt he could have had a penalty for a trip before Harrison Foulstone saw his effort blocked as United did their best to break down a stubborn Crewe side.
Derek Geary introduced the attacking trio of Sammy Ompreon, Antwoine Hackman and Reon Potts to try and force an opening and all three began to cause Crewe issues with the fresh legs.
But with 10 minutes of the game to go it was Crewe who had the best chance to steal a win. Some sloppy defending allowed Linton in on goal but Sam Kelly made a big save before Jean Belahouan did brilliantly to get back on the line to deny Abdul Djalo on the rebound.
The block became even bigger in stoppage time when Ompreon broke forward, played a ball through to Maguire and the midfielder did the rest with an emphatic finish past Sam Booth.
Potts had a chance straight after to seal it but saw his low effort saved in what was the last attempt of a game United deservedly won in the end.
Blades: S. Kelly, Williams, Anderson, Neal, Belehouan, Foulstone, J. Kelly (Ompreon 74), Maguire, Gaxha (Hackford 64), Brunt, Greaves (Potts 62).
Unused: Chapman, Clarke.