Western Force found some much needed confidence - and a potential backline superstar in James O'Connor - as they pipped the Chiefs 22-21 in a Super 14 thriller at Subiaco.

Established Wallaby Matt Giteau slotted a penalty after the siren to keep the Force's slim semi-finals hopes alive.

But, making his run-on debut at just 17 years, O'Connor belied his inexperience and diminutive frame with a big performance.

The teenager was great in defence and attack and set up the third Force try for James Stannard after early scores to Matt Hodgson and Drew Mitchell.

Second half tries to Lelia Masaga and Stephen Donald and a field goal to Callum Bruce put the Chiefs in sight of a sixth successive win - until the after-the-siren penalty gave the Giteau the chance he did not waste.

Having won just one of their last four outings, Force coach John Mitchell made wholesale changes, including elevating O'Connor and recalling Matt Hodgson into an unaccustomed number eight role.

It was Hodgson who made the earliest impact, charging down Richard Nutbrown's defensive clearance to plunge over.

But after another Force prospect Kieran Longbottom was given his debut following an horrendous thumb injury to Pek Cowan, it was O'Connor who showed why he has already been compared to Tim Horan and Giteau.

Mitchell's try on 11 minutes put the Chiefs on the back foot, with their natural fluency - and Donald's place kicking - both going missing.

Trailing 14-3 at halftime and with their finals ambitions sinking, it took just two minutes of the second half for Sitiveni Sivivatu's step and hand off to put two Force defenders out of the equation, with a raking pass finding Masaga wide.

That set up an amazing second half, with O'Connor showing his class, the Chiefs showing their desperation and the Force showing the fight lacking in recent weeks.

Inside the last 20 minutes, O'Connor's step and break led to Stannard's first Super 14 try.

But, from the next phase, Masaga topped O'Connor's midfield break with a fabulous stepping run, allowing Donald to stroll under the posts.

Donald's penalty five minutes later brought the deficit back to one, prompting a crazy passage of play with minutes remaining.

With Nick Cummins and Troy Takiari laid prone with injuries, the Chiefs somehow failed to score - until Bruce's sweet strike with three minutes remaining.

But the Force would not lie down, with bursts from Shepherd and Sitaleki Timani putting the home side in range of the line, Shepherd eventually diving over in the corner.

Referee Marius Jonker disallowed the try due to a knock on, but a penalty advantage for an earlier offside allowed Giteau to slot the winning points.

Force coach John Mitchell said the win redeemed a season that had slumped alarmingly before last week's bye.

"We got the result and we probably deserved to in the way we played, our intentions were good and we were positive," Mitchell said.

"We got back to some of the disciplines that have enhanced our attack, defensively we were well organised and there was great desperation.

"The group re-established themselves as rugby players, re-established themselves in the community and re-established themselves as a team."

Skipper Nathan Sharpe said given the in form opposition and changes to personnel, he was delighted with the result.

"For the boys to throw everything at a team like the Chiefs who are obviously a team with a chance of reaching the semi-finals, was very impressive," Sharpe said.

"Couple with that the preparation this week, with the injuries and what not, I am very proud of the boys."

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