Western United will contest the Far West football A grade grand final after twice battling back to defeat Koonibba by 33 points at Penong on Saturday.
A see-sawing affair saw the Roosters lead at quarter-time and three-quarter time, only for Westies to recover to win 13-13 (91) to 8-10 (58).
Jarden Stoetzer continued his remarkable season as he notched his 100th goal on his way to four for the day, and he was joined by teammate Tom Woods.
Tyron Winfield and Vaughan Day kicked three apiece for Koonibba.
The Roosters came out firing to kick five unanswered goals for a 35-point lead, before a late Western United goal reduced the deficit to 29 at the first break.
That goal got the ball rolling for the Tigers as they took command of the second quarter with seven goals to one for a 12-point advantage heading into half-time - including three Jesse Chandler majors.
Westies could only add two behinds in the third term as two goals gave the Roosters a one-point lead at the final break.
It set the stage for a fascinating final term, which was dominated by Western United.
The home side kicked five goals while keeping Koonibba scoreless to advance.
Westies will face the winner of Saturday's preliminary final between Koonibba and the Blues.
Meanwhile, Koonibba reached the B grade grand final thanks to a 69-point win over Western United.
The Roosters were too good throughout to grab a 15-15 (105) to 5-6 (36) victory.
They kicked the first two goals of the match to lead by six points at quarter-time, before adding the only goal of the second term.
Clear by 12 points Koonibba pushed further ahead, with four goals to two in the third term extending the margin to 26 points.
The Roosters powered through the final quarter with eight goals to march into the decider where they will face either Western United or Thevenard.
Shaquille Kruger led the way with four goals.
Koonibba held on to win the under 17s clash against Thevenard by 11 points.
It was a match on contrasting fortunes before and after half-time, with the Roosters winning 10-2 (62) to 7-9 (51).
Maliki O'Loughlin was exceptional in kicking five goals for the Roosters.
Koonibba kicked three goals to one in the first term and with four unanswered goals in the second led by 34 points at the main break.
The Magpies had the better of the third term with three goals to two for a 23-point deficit, but five behinds hampered their progress.
Koonibba crucially kicked the first goal of the final quarter before Thevy grabbed the next three, but it was not enough.
Thevenard has the double chance against Western United this Saturday.
The Blues won the under 14 clash with Koonibba by 77 points.
They were on top from the outset for a 13-12 (90) to 2-1 (13) result.
Parker Fawcett kicked four goals and Kade Maddigan added three for the Blues.
The Blues kicked the only four goals of the opening quarter, although with two majors Koonibba slightly reduced the gap at half-time.
It was a blue wave from there as the Blues kicked four goals in each of the final two quarters.
Koonibba meets Western United in the preliminary final this Saturday.