Wests snatched premiership favouritism and the first berth in this year's grand final with a thrilling 28-26 victory over minor premiers Helensburgh in Sunday's major semi-final at WIN Stadium.

Having played only sparingly as a combination this season, Wests halves Daniel Holdsworth and Nathan Dureau - back from a four-week injury lay-off - proved the Devils' trump card in the victory both picking up tries and coming with big plays in a match that came down to the dying seconds.

Jason Ryles' men scored five tries to four but it was a victory built on defence with the Devils constantly called on the defend their line in the second half including four straight sets in the final five minutes.

"It was defence that won us the game," Ryles said.

"The first half was pretty disappointing - some of the one-on-one misses we came up with on the try line but the Burgh are a quality team and you've got to be on your game for [the] 40 minutes of both halves.

"It was just good we were able to turn it around and hang in there when times got tough in the second half.

"To grind it out when the game was on the line was really pleasing."

After leading 12-0 early, by means of tries to Marty Cramp and Holdsworth, the Devils were left with 12 men when prop Greg Reh was dispatched to the sin-bin for his part in a scuffle.

Helensburgh took full advantage with Ben Ryan and Steve McCallum crossing in the space of three minutes to level-up at 12-all before a Grant Smith penalty goal gave the Tigers a 14-12 lead. The extended their lead when Guy Gellatley swooped on a dropped ball from Mitch Porter but the Wests custodian made amends almost instantly, scoring a try at the other end, keeping it tight at 20-18.

Wade Standford crossed after the break to retake the lead for Wests before Dureau's try from a 40-20 from Holdsworth gave them an eight-point buffer with 19 minutes to play.

The Tigers got within two with a 73rd-minute try to Grant Smith and had a chance to send the game to golden point when they were awarded a penalty within range with less than three minutes on the clock but instead rolled the dice looking for a try.

A disappointed coach Ryan Powell accepted the decision, challenging the Tigers to bounce back.

"You can go for the draw and try to battle it out, in a grand final you'd do that, but today my thoughts were: either win the game or you don't deserve to win it and you come back next week," Powell said. "That was the gamble I took and we weren't good enough, so we'll come back next week.

"That's fine by us, we'll play Thirroul and earn our spot.

"We're not ready to go into a grand final after that display so that probably won't hurt us."