The KwaZulu-Natal outfit dominated the opening stages of the encounter with striker Thandolwethu Ngwena coming close before opening the scoring with a close-range finish in the 42nd minute.
However, the visitors pulled one back through a deflected strike from Tshepo Mokoane just two minutes later after the home side seemed to relax and anticipate taking a lead into half-time.
Both teams came out firing in a frantic start to the second half, but Babina Noko grabbed a deserved lead as they added a second goal in the 52nd minute after AmaZulu failed to clear their lines.
The situation seemed to go from bad to worse as United's Ellis Rammala smashed home a loose ball into the net from outside the box after a mazy run.
Usuthu pulled one goal back as Ngwenya grabbed his brace from a near post header before veteran defender Taariq Fielies equalized for the Durban based outfit in the 58th minute.
Neither side could find a winner within regulation time, forcing the match into extra time, with Vuyo Letlapa breaking the deadlock from a tight angle after AmaZulu failed to deal with a speculative cross.
Sekhukhune maintained their discipline to maintain a 4-3 lead and qualify for the MTN8 semi-finals alongside Orlando Pirates and Mamelodi Sundowns.
