The Blues have arrested their alarming form slump to clinch a place in the Super 14 semi-finals, ending the Western Force's slim chance in the process with a commanding 33-6 win at Subiaco Oval.

On the day Force coach John Mitchell agreed to stay in Perth for another four years, he would not have liked what he saw, with basic errors infusing much of the Force play to end an improved season on a dowdy note.

By contrast, David Nucifora would have been delighted with the win, with the power of his pack at the breakdown and in the scrum grinding the home side into submission.

After two Luke McAlister penalties to one from Giteau edged the Blues ahead, the quality move of the night saw Rudi Wulf cross to prove his place ahead of Joe Rokocoko was warranted.

After that, referee Jonathan Kaplan's whistle was the most dominant force - closely followed by the Blues front-row, who claimed three tries between them to clinch what could be a crucial bonus point.

John Afoa's first-ever try rewarded the pack, Keven Mealamu's breakaway embarrassed officials - after a quick throw was taken with a different ball, which should have ruled it out - before Tony Woodcock's plunge finished the contest.

With talk from the Force of putting on their best performance to end the year, it was in fact one of their worst.

Although driving rain did not help either team sparkle, errors from hand and boot peppered Force play, with Matt Giteau one of the main offenders.

Passes the Wallaby star would normally complete in his sleep were either high or low, and missed kicks were also indicative of his ordinary night.

The Force knew before kick-off their slim semi-final hopes had been all but extinguished by the Chiefs' shock win over the Crusaders.

And their first scrum of the night set the tone for the night, with three resets ending with a penalty against Troy Takiari - one of four similar offences over the night.

After McAlister, returning from a broken cheekbone, slotted the first points, the Blues pack continued to hammer into every breakdown, denying the Force an inch of unearned territory, despite Giteau's equalising kick.

Moments after McAlister edged the visitors in front with his second goal, Nucifora's backline kicked into gear to lethal effect.

Isa Nacewa made the initial break, before McAlister took over, perfectly timing his offload to Isaia Toeava, who was collared on the goalline by Drew Mitchell.

The recycled ball found its way to Rudi Wulf and his dummy easily fooled the Force defence.

Giteau's second penalty on the stroke of half-time gave the home fans some hope, but it did nothing to relieve the pressure - with every half break snuffed out with ruthless defence.

When Afoa barrelled over with ten minutes to go, the result was beyond doubt, and even the officials seemed to have lost interest when Mealamu's score was deemed legal, despite the fresh leather provided by the ball boy.

Woodcock's late effort then gave the front-row union a treble - and plenty for Mitchell to work on during his extended stay.

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