The NFD newcomers dumped PSL club AmaZulu out with a 3-2 win in uMlazi on Saturday.
It was the second-division strugglers’ second big league scalp of the cup, having knocked out Polokwane City in the earlier round by the same score.
Since that win, they have been unbeaten in the league, picking up a draw and two wins to pull away from the relegation zone.
Kamohelo Mahlatsi and Jesse Donn combined for Clint Fredericks’ headed opener in the seventh minute.
And Duncan Adonis made the most of the space created by veteran Franklyn Cale to boot in the second.
Fredericks then turned provider for Mahlatsi to slot in from close range in the 48th minute for an insurance goal.
Mabhuti Khenyeza pulled one back for Usuthu and Ovidy Karuru got AmaZulu’s second in the 90th minute.
Meanwhile, Steenberg United went down to 2-0 at Baroka and Stellenbosch tested Kaizer Chiefs before the Soweto giants booked their place in the last eight with a 2-1 win.
A goal in each half from Erick Mathoho and Leonardo Castro gave Steve Komphela’s manne a cushion before Chiefs were made to endure a nervous finish when defender Alan Robertson flicked a header in from a corner in the 77th minute.