Harrop drifted easily past Brady to slip in Tom Pratt but the forward's effort was met with a good stop by Johnstone with his foot.
United capitalised to deservedly level before the break as a lung-busting run by Tunnicliffe allowed him to to feed King inside the box and the Norway international buried an emphatic finish past Dibble. Petrucci really should have scored soon afterwards, following an electrifying run by King that started inside his own half, but the captain was unable to scramble the ball home despite having two bites of the cherry.
The second half followed a similar pattern but with Warren Joyce's side displaying more composure in front of goal. Jesse Lingard tucked home clinically to earn Tunnicliffe another assist and then saw his driven cross evade Petrucci at full stretch. When King twisted and turned to blast in another shot, Dibble managed to deflect it onto the woodwork.
Lingard and Macheda whistled attempts off target but a third goal duly arrived courtesy of Marnick Vermijl, who won possession high up the right flank and despatched an unerring finish past Dibble to hand the Reds some breathing space. Another example of King's exceptional acceleration allowed Brady to steer wide but the Norwegian was guilty of a wasteful finish after being set clear by Petrucci.
King had another attempt parried away by Dibble but the keeper was under siege and had little chance when Petrucci did well on the left to tee up his pal Macheda and a low shot found the back of the net. Lingard stung the hands of Dibble as the keeper continued to do his best to repel the threat, King dragged wide of the far post and Jordan Melia hacked a Petrucci shot off the line as United continued to dominate and were good value for the four goals.
United: Johnstone; Vermijl (Thorpe 79), M.Keane, Wootton, Blackett; Brady, Tunnicliffe, Petrucci, Lingard; King (James 90), Macheda. Subs: Veseli,