Having beaten Darlington in the last round the Blades will have to wait until 31 March before they learn who they face in the last four.
The other teams contesting the semi-final are Wrexham and Sheffield Wednesday who met at the Recreaton Ground.
Meanwhile the Reserves beat Tranmere Rovers 2-1 on there own ground yesterday. A Robinson penalty put the home side ahead after 36 minutes.
United, however levelled two minutes later, Ashley Sestanovich went bursting down the wing and realeased Jonathan Forte who beat the keeper from close range. The visitors then won the game with a Robertson header after Ian Ross provided the cross.
Former Blades Iffy Onuora and Shane Nicholson were in the Rovers line-up.