The Roosters kept their own finals hopes alive but are relying on North Queensland, who face Penrith on Saturday, or Canberra, who travel to Cronulla on Sunday, to lose to ensure they make the top eight.
But whatever happens later on in round 27, the Roosters will take great delight in being the team to put a knife through South Sydney's season in front of 36,263 fans at Accor Stadium on Friday.
Souths became the first side in the NRL era to lead the competition through the first 11 rounds and go on to miss the finals.
"From where we got to and the pre-season we had, it's disappointing," Rabbitohs coach Jason Demetriou said.
"That performance probably summed up our back end of the year - we were just not quite nailing things.
"Our effort was outstanding but we just didn't capitalise on the opportunities and then leaked points.
"I've got to hold my hands up there. I'm the head coach, it's on me."
The possible causes for Souths' dramatic implosion came to the surface over the last fortnight.
The departure of assistant coach Sam Burgess last week after he raised concerns over Demetriou's handling of superstars Latrell Mitchell and Cody Walker was a convenient flashpoint.
But there had been earlier warning signs Souths, who reached last year's preliminary final, were on the wane.
In May, bookmakers installed the Rabbitohs as favourites to take out the title but absences for key forwards including Tom Burgess, Jai Arrow and Tevita Tatola hampered their premiership tilt.
Perhaps most important of all was the calf injury picked up in Origin camp that condemned Mitchell to more than two months on the sideline.
The fullback was again absent on Friday, due to suspension. It didn't help Souths' fortunes that his replacement Blake Taaffe was taken off with a concussion inside the opening 20 minutes.
The Roosters have had their own fair share of issues this year but when they needed their men to deliver, they did.
Kiwi centre star Joey Manu and prized winger Daniel Tupou were both ruled out of Friday's game and it was fitting that their replacements, Corey Allan and Junior Pauga, gave the Roosters a 10-6 lead at the break either side of a Cameron Murray try for the Rabbitohs.
Maligned hooker Brandon Smith came up with some great last-ditch tackles before Roosters captain James Tedesco finished a classy move in the 45th minute to open up a second-half buffer.
While Michael Chee Kam hit back for the Bunnies, tries from Billy Smith and Sam Walker helped kill off South Sydney's season.
The Roosters now need results to go their way to keep their own campaign alive.
"I don't know if I'll be sitting down watching every bounce of the ball tomorrow (for the Cowboys game) ... I'll let someone else decide how our season goes," coach Trent Robinson said.
"It was do or die for us.
"To execute a gameplan under those conditions takes a really clear team. You can see that we've got a bit of backbone to us."
