Star upset top qualifier and defending champion Marist 34-20 in its semifinal at Rugby Park and Blues beat the Eastern Northern Barbarians 15-8 at Les George Oval.
Star had a good start by building phases and pressure, its kick-and-chase game leaving it at the right end of the field.
It kicked a penalty in the first 10 minutes, followed by two converted tries to lead 17-0.
Star had a prop sent to the sin bin and Marist kicked a penalty before halftime.
Marist strung some plays together in the second half and scored a converted try before Star stretched the margin to 27-10.
With 10 minutes to play, Marist had closed the gap to 27-20 but Star had the final say.
Star assistant coach Chris McIlwrick was especially pleased with his team's tackling.
"We produced our best defensive effort of the season. We kept Marist at bay for multiple phases and nullified their big ball-carriers.
"Flanker Jordan Stewart was outstanding and sent another big statement to the Southland selectors.
"The whole forward pack stood up and our utility back Jay Ngalafe had the best game since arriving back from Italy."
Blues deserved its 15-8 victory over the Eastern Northern Barbarians.
Blues led 10-3 at halftime and both teams scored a try each in the second half.
Barbarians co-coach Mark Hourston said it was great effort from his team despite coming up short.
"We came up against a very well organised Blues team that we couldn't crack at the crucial time in the game."
Wyndham will host the Ack Soper Shield competition final this weekend against the red-hot Midlands.
Wyndham won its semifinal against Te Anau 22-10 and Midlands beat Edendale 33-15.
The Midlands victory was comprehensive; Midlands possessed a dominant scrum and a smothering team defence.
Hard-running loose forwards Sam Brame and Tim Watt punched holes in the Edendale line, while fullback Henry Earland was confident and comfortable on defence and a constant threat on attack.
Midlands led 17-3 approaching halftime when it had a man sent to the bin and Edendale scored from the resulting penalty.
Edendale was back in the game at 17-8 but, early in the second half, it had two men sent to the bin in quick succession and Midlands scored two more tries.
Wyndham came from behind to beat Te Anau 22-10.
Te Anau was leading 10-8 for the first 70 minutes of the game but a controversial penalty try and a late converted try booked the boys in blue a home final.
- John Langford