Dunedin Technical survived a second-half fight-back from Western to register a hard-fought 1-nil victory at Tahuna Park.
After an even start to the first half, Technical struck the decisive goal in the 20th minute.
Some constructive play by Tech midfielder Victor Da Costa led to a good attack down the right flank, with Aaron Burgess driving a dangerous ball across the Western penalty area.
What appeared to be a straightforward conversion was made twice as hard with an initial weak shot on goal by Alistair Rickerby.
The Western defender's partial shot block bounced fortuitously for Rickerby to bundle the ball into the net at his second attempt.
Tristan Prattley was a stand-out performer for the home side.
Like many teams chasing the game when behind, Western lacked the final killer pass or cross to convert numerous half-chances.
Technical hung tough to its slender lead and bodies went on the line to save goals.
"That's cup football," Technical coach Mike Fridge said.
"You just want to be in the next draw.
"We could have taken another goal or two lead at the break, and Western threw everything at us in the second half but we held our nerve and kept a clean sheet."
Technical will be looking forward to a favourable draw on Tuesday for the quarterfinals, and will need to make some solid improvement in its next cup match to progress further.
In Saturday's premier league fixtures, Mosgiel moved into fourth place with a narrow 2-1 home win over Roslyn Wakari.
Grants Braes' season continues to suffer with a heavy 7-1 away defeat by Queenstown Rovers, after having a player sent off just before halftime.
