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.
Orlando Pirates coach Abdeslam Ouaddou has defended his team's performance after they needed extra-time to see off Durban City 2-1 in a closely contested MTN8 quarterfinal.
Cassius Mailula came off the bench to net the winner as Mamelodi Sundowns beat Polokwane City 3-2 after extra-time to advance to the MTN8 semi-finals at the Lucas Moripe Stadium on Saturday evening.
Ghampani Lungu produced a moment of magic in the final minute of extra-time as Orlando Pirates secured a dramatic 2-1 victory over 10-man Durban City at the Orlando Amstel Arena on Saturday to keep their MTN8 title defence alive.
Polokwane City skipper Puleng Marema is determined to make the most of the club's MTN8 opportunity as he continues his quest for a first major trophy in South Africa's top flight.
