Sydney Roosters representative back-rower Aidan Guerra is likely to be given the onerous task of shutting down in-form South Sydney halfback Adam Reynolds when the bitter arch-rivals clash in Friday's NRL preliminary final.

In his return from a one-match suspension for a chicken-wing tackle, Guerra managed a first-class lockdown job early on North Queensland halfback Johnathan Thurston in Friday night's semi-final, smothering the superstar playmaker as the premiers raced to a 30-0 first half lead.

But with Guerra benched, Thurston was allowed to work his magic to inspire a dramatic Cowboys fightback, the Roosters needing a field goal to eventually triumph 31-30.

It set up an epic clash with Souths for a spot in this year's grand final.

Guerra said his side would likely be punished more emphatically against the Rabbitohs should there be a repeat performance.

"We've got to learn from that lapse we had and make sure that doesn't happen again," the Queensland representative said.

"But we definitely believe that if we feel momentum against us we can get back to what we do and we'll be right.

"JT is such an elusive player, everyone seems to give him more time but I learnt to hopefully try and use that against him.

"You have to try and take time off a player of that quality and at times we did that but there was that time when we didn't.

"We switched off there and you can't do that against a player of that quality.

"We have to make sure we don't do that again [against the Rabbitohs halves]."

To many the match is the decider that's come two weeks early and is a game 76 years in the making.

Incredibly, the two foundation clubs, who can boast 33 premierships between them, have not faced off in a play-off match since a semi-final on August 27, 1938.

The Roosters triumphed 19-10 that day before losing to Canterbury in the grand final a week later.

After their qualifying final thumping of Manly, Souths have moved into premiership favouritism but will start only slightly ahead with the bookies in what will be the last game in the NRL for Sam Burgess or Sonny Bill Williams, who are both bound for rugby union.

Burgess has been a key figure in Souths' surge to their third successive preliminary final.

So, too, has the late season form of Reynolds, who has combined spectacularly with Luke Keary in the halves since the latter returned from a preseason pectoral injury.

Souths coach Michael Maguire says the key to Reynolds and Keary's on-field relationship is their mateship off the field.

"Those two are really close friends, they get along well and they enjoy playing footy," he told Triple M on Sunday.

"They kick for each other, they feed off each other and their talk has been great.

"I think that has been a great strength of Adam the past six weeks."

In the second preliminary final, Penrith play Canterbury at ANZ Stadium on Saturday night. AAP