Wests rose to the occasion with a dominant second-half display to knock reigning premiers Thirroul out of the title race on Saturday at Collegians Sports Centre.
Proving their poor late-season form was just a smokescreen, the Devils broke a 4-4 half-time deadlock with four tries before withstanding a late fightback to prevail 20-14.
They would've won by a more comfortable margin if goalkicking centre Brett Kelly hadn't missed all five of his conversion attempts.
Thirroul scored two tries in the final 10 minutes and were still desperately seeking an equaliser in the dying moments.
But the Devils hung on, booking a date with Helensburgh in this Sunday's final.
Despite losing five of their last six regular season games and finishing fifth, Wests have caught a whiff of that familiar premiership scent.
"We're just in there one week at a time at the moment," Devils co-captain Matt Clarke said.
"We had dramas at the club and we're fighting our way out of it one game at a time.
"We were down and out last week [in the elimination final against Shellharbour]. If we lost, we were gone, so it's great that we're still in there fighting.
"There's definitely a belief that we've got the team to keep winning. If we play like we have the last two weeks, any team can win on the day."
Neither team would budge in the first 20 minutes and errors were kept to a minimum.
The stalemate was finally broken with Butchers centre Reece Jaegar's try, but Wests hit back with winger Zac Hill's try.
The Devils drew first blood after half-time with fullback Brad Scott's try and stretched the margin to 12-4 with Hill's 90-metre intercept try.
They added two more tries for a 20-4 lead before the Butchers made their late charge.
The Devils forwards led the way, particularly their tireless hooker Clarke.
"The forwards stood up today, so that was really good," he said.
The Butchers had a team capable of claiming back to back titles, but they let themselves down at crucial stages of the second half and were left to ponder what might have been.
"That's probably the best start we've had all year," coach Phil Ostwald said.
Wests beat Shellharbour 28-20 in a see-sawing reserve grade clash and Norths downed North Wollongong Surf 32-20.
Dapto accounted for Corrimal 36-24 in the under 18 game, while Helensburgh defeated Corrimal 22-4 in the women's match.