Thirroul seized their first minor premiership since 2008 with a come-from-behind 20-18 victory over Wests in a thriller at Gibson Park on Saturday.
It was a gutsy performance that typified the Butchers' season and firmly established them as favourites for the 2014 title.
The Butchers trailed 18-4 just after half-time following an intercept try to Devils flyer Jordan Harty but - riding a wave of support from a huge Old Boys Day crowd - fought their way back to score the final three tries and snatch the result.
The loss relegated Wests to the minor semi-final after Helensburgh also notched a come-from-behind 32-20 win over Collegians to seal a top-two spot.
Wests will play defending premiers Collies at Collegians Sports Centre on Sunday with the northern derby major semi-final between the Tigers and the Butchers the following weekend.
Butchers coach Nathan Fien said the win showed what it took to win finals matches.
"Going into the game we knew we were going to find out what our footy side was about," Fien said.
"I challenged the boys and asked them how we wanted to be seen as a football team and the way the result panned out it showed the side's got a lot of will to win.
"They definitely play for each other. It's a never-say-die attitude and that's what this team's about. At this time of year in finals footy that's what you're going to need to grind out footy games."
Thirroul took an early lead through Pat Franks when Harty spilled a kick from Fien, giving the Butchers a 4-0 buffer after just seven minutes.
It was their last joy of the half as Wests, through big men Jason Ryles, Greg Reh and Justin King, rolled through the middle to keep the home side on the back foot.
The pressure told when the prolific Mitch Porter scored to level it 4-4.
Porter slotted a penalty goal soon after to take a 6-4 lead for his side. Porter scored again three minutes from the break with his sideline conversion pushing the lead out to 12-4 at half-time.
Harty's runaway effort straight after the resumption gave the Devils a seemingly unassailable lead after Porter slotted his second penalty shot at goal to lead 18-4 with 27 minutes to play.
After absorbing the onslaught, the Butchers scored through Luke Gallaghar in 60th minute to give themselves a sniff and when Luke Dodge scored five minutes later to cut the lead to 18-16 the comeback was in full swing before Sam Clune scored the match winner.