The Roosters crushed the play-off hopes of arch rivals South Sydney in the final game of day one at the Downer NRL Auckland Nines, emerging from a scrappy affair as 14-7 winners.
The win means the Roosters will advance out of the Hunua Pool with either a win or draw against the Raiders on day two.
First-half efforts from Paul Momirovski and Connor Watson gave the Roosters an 8-0 lead at the conclusion of the opening nine minutes.
But Souths hit back thanks to hooker Damien Cook, who broke through just after the break to score the 100th try of day one before converting to put his side back to within a point.
However a Daniel Tupou try and subsequent Latrell Mitchell conversion inside the final minute ensured the Roosters held on to record a vital win.
Roosters 14 (Paul Momirovski, Connor Watson, Daniel Tupou tries; 1 goal) def. Rabbitohs 7 (Damien Cook tries; 1 goal). Half-time: 8-0.