The Blades are through to the final of the U18 Professional Development League after an impressive play-off victory over Premier League opposition at Vicarage Road on Saturday.
After goalkeeper Aaron Ramsdale had saved a first-half penalty, Travis Binnion's youngsters produced a fantastic second-half display with Jordan Hallam firing the visitors in front from the spot before further strikes from Connor Hall and Nathaniel Crofts set up a final at the winners of Nottingham Forest and Charlton Athletic.
The size of the Blades task was a big one against a Hornets side that were unbeaten in 15 matches and who had only conceded two goals in that spell, but United showed no fear and Hallam had a goal bound shot blocked within the first two minutes. Watford responded with Obi flashing a shot across goal.
It was an open affair, Smith had a shot saved by goalkeeper Thomas before defender Cummings saw his header cleared off the line, at the other end Ramsdale stood big to deny the hosts after a slip in the visitors' rearguard.
Having had appeals for a penalty waved away on the half hour following what appeared to be a push on Hallam, the Hornets were awarded a soft one on the stroke of half-time following a foul by Semple, but Ramsdale was at his best diving away to his left to deny Obi.
The Blades started the second period confidently and only another save from Thomas prevented Smith after good work by Crofts down the right, but in response Watford forced Ramsdale into a double save, saving at the near post before smothering the rebound.
But in the 65th minute, the game appeared to have swung decisively in favour of United, as substitute Mason saw red following foul play in the penalty area. HALLAM stepped and converted with his effort going in off Thomas after his spot-kick had hit the post.
Ten-man Watford replied within two minutes though as FOLIVI fired home after Obi had cut the ball back from the byline having got beyond the Blades defence. But United weren't to be denied and in the 71st minute, HALL headed past the onrushing Thomas to restore the visitors' advantage and this time they were in no mood to surrender.
In the very next attack, United made sure of the victory as Hall went from scorer to provider, getting the better of his man before squaring to the outstanding CROFTS who calmly slotted home, much to the delight of United players and staff.
The Blades managed the game excellently in the final 15 minutes to net a deserved success against top flight opposition and set up a final showdown against either the Reds or the Addicks.
Blades: Ramsdale, Charlesworth, Cummings, Semple, Brown, Crofts (Ford 89), Gordon, Gilmour, Smith (Litchfield 66), Hallam (Slater 84), Hall.
Unused: Warhurst, Adebowale.