Ben Stokes' maiden Test half-century kept the Ashes barely alive for one more day as England produced some last-ditch defiance in defence of the urn at the WACA.

After Alastair Cook was out for a golden duck, and then Kevin Pietersen holed out in the deep, England's chances of even delaying Australia into the last day of the third Test were improbable.

But in notional pursuit of a world-record 504 to win, following some brutal hitting from Shane Watson (103) and George Bailey in Australia's 369 for six declared, Stokes (72 not out) and Ian Bell (60) responded with a counter-attacking stand of 99 in a stumps total of 251 for five.

From 2-0 down already, England remained obvious candidates to lose the Ashes here - but at least their fifth-wicket pair had at last put some unaccustomed pressure on the home attack.

Bell's half-century contained five fours and a six, ramped over third-man off Ryan Harris, and all-rounder Stokes' first 50 - in his second Test - came from only 69 balls.

He completed it with a straight-drive off Watson, for his eighth four, in the over after Bell had succumbed caught-behind off Peter Siddle, when 'Snicko' - but not 'Hot Spot' - persuaded Tony Hill to overturn Marais Erasmus' initial not-out verdict.

England's unlikely rearguard got off to a horror start when Cook was bowled by a near unplayable delivery from Harris.

Michael Carberry and Joe Root hinted at lasting resistance in a second-wicket stand of 62. But with them gone, once Pietersen's fancy attempt to bring up his 50 with a six off Nathan Lyon instead ended with a steepling catch at long on for Harris to make it 121 for four, it seemed all hope was spent for England.

After the instant shock of losing their captain for a golden duck, beaten on the defence by a beauty which clipped the off bail, they had already shown some appetite to go down fighting at least.

Yet on a pitch crazy-paved by wide cracks after being baked by 100 degree heat for three days, it seemed unthinkable they could eke out 160 overs - let alone top 500 in the process.

Carberry and Root responded well in adversity, until the left-hander - for the second time in the match - was undone by a change of angle.

This time, Watson went round the wicket and pinned him lbw on off-stump.

Root continued to defend stoutly until he squeezed an edge behind from a very full Mitchell Johnson delivery, brilliantly caught one-handed by a diving Brad Haddin.

DRS could not prolong Root's 88-ball vigil for just 19 runs.

Watson earlier had free rein to attack on another stiflingly hot morning, albeit with more cloud cover, and took rich advantage with a 106-ball century containing 11 fours and five sixes.

He was merciless against Graeme Swann, signalling Australia's intent by hitting the off-spinner for two fours and a six from the last three balls of the day's first over.

Swann's final over, before being diplomatically replaced by Root, was then smashed for 22 by Watson - an initial four followed by three sixes into the Prindiville Stand.

Steve Smith's wicket, caught in the leg-side deep off Stokes, hinted at a modicum of respite.

But Watson, who had dominated the fourth-wicket stand of 78 in 16 overs and was on just 29 at start of play, was in the mood for more mayhem.

A direct hit by Root from mid-off would have run him out for 51, and in the 90s Tim Bresnan clung on to a catch at long-off only for the momentum of the ball to carry him over the rope.

Watson therefore completed his fourth Test hundred, and Australia's seventh to England's none in this series, taking just 28 balls over his second 50.

But a double-play from Bresnan's first delivery with the second new ball brought a bizarre end to Watson's fun.

Bell dropped a miscued skier in the off side only for the ball to drop within Bresnan's reach in his follow-through, from where he threw down the stumps with Watson still mid-pitch.

There was still time for more haplessness in the field from England, and for Bailey to club James Anderson for a world record-equalling 28 in an over, before Michael Clarke ordered the declaration.

He had budgeted, it seemed, for a slow kill but plenty of scoreboard insurance too - and thanks to Stokes and Bell, he was perhaps wise on both counts.