United's U18s made it three consecutive victories in the Professional Development League on Saturday, with a last-gasp success over Crewe Alexandra at the SteelPhalt Academy.
Reon Potts opened the scoring at Shirecliffe, but the Railwaymen looked to have snatched a point late on, only for Cameron Hawkes to net with the last kick of the contest.
The Blades had the first chance of the game when Hawkes picked out the run of Greaves, who for his effort to be saved at the near post. At the other end, the Alex responded with a chance of their only, hitting the crossbar with a header.
As the interval drew closer, it was the Blades turn to rattle the woodwork, as Greaves's deep corner was met by Lawrence.
After the break, just moments after Crewe had hit the upright again, United broke the deadlock. Greaves worked his way into the box and his effort was too hot to handle and POTTS followed up to score.
Crewe refused to lay down though and only a fantastic double save from Warhurst prevented a certainly goal with 20 minutes remaining.
But in the 84th minute, the visitors did restore parity and looked to have snatched a point as they converted a swift counter attack despite the best efforts of Warhurst.
It seemed that the points would be shared, until the final minute of added time, a curling free-kick from the left from Mason found the run of skipper HAWKES and his header beat the goalkeeper to spark celebrations on and off the pitch.
Blades: Warhurst, Brown, Foulstone, Cutts, Gordon, Belehouan, Hawkes, Broadbent, Lawrence (Mason 76), Greaves (Kelly 86), Potts (Harriott 79).
Unused: Ompreon, Dewhurst.