The upsurge in tempo carried over into the second half, with United's approach full of vim and vigour, yet once again quickly faded out in the face of Reading's physical, resolute approach.
Nemanja Vidic's heavily deflected header needed to be cleared off the line by Noel Hunt, Federici comfortably parried away Anderson's right-footed effort and Nani scuffed a shot narrowly wide as United gradually cranked up the pressure.
By the time the Portuguese arrowed another left-footer wide from distance, Robin van Persie had been introduced in place of Young as Sir Alex Ferguson sought to stave off the growing spectre of a replay. Sure enough, the dam broke with 21 minutes remaining.
Patient approach work between Cleverley and Valencia culminated in Nani controlling with his left and firing a 15-yard right-footed effort into Federici's far corner. Joy and relief mingled as Old Trafford roared its delight.
That bellow had barely subsided when Reading almost silenced all-but the small pocket of travelling supporters, as Adam Le Fondre hooked in a shot which required a fine reaction stop from De Gea. It proved a pivotal moment in the game, as United struck decisively just seconds later.
Again Nani was involved, speeding down the right flank and curling in a magnificent cross which Chicharito reached ahead of Federici to nod