Joe Root helped England extend resistance into a final day in their unequal battle to save the second Ashes Test at Adelaide.

Root (87) and Kevin Pietersen (53) contributed England's first century stand of the series, and then Matt Prior and Ben Stokes dug in manfully too to help keep four wickets still intact at stumps.

The tourists' mission improbable therefore sneaked tenuously past half-way but, even with some poor weather forecast for Monday, the escape route appeared distant.

England struggled to 247 for six at stumps, in notional pursuit of a world-record Test victory target of 531 after Australia declared on their overnight 132 for three.

Root and Pietersen joined forces after both openers went early, trying to hook their team out of trouble.

From 20 for two, the second-wicket pair put on 111, and Root emerged with particular credit despite eventually succumbing to another Ashes bogey number 13 short of a hundred.

The young Yorkshireman, whose occupation of the number three position came into question after just one opportunity there when he was out slog-sweeping Nathan Lyon in the first innings, demonstrated his suitability beyond reasonable doubt against Mitchell Johnson et al.

He and Pietersen withstood two express spells from Johnson, and Root refused to budge for almost four-and-half hours until he inside-edged Lyon on to his box and could only watch in horror as the ball looped sufficiently for an alert and agile Brad Haddin to dive into the off side to get his wicketkeeping glove underneath it.

Australia captain Michael Clarke judged that he had put runs out of the equation, and could therefore use a variety of tactics - including the expensive leg-spin of Steve Smith - to try to eke out wickets.

None succeeded for 36 overs against England's third-wicket pair. But then Pietersen pushed crookedly forward to his nemesis Peter Siddle and saw a faint inside-edge divert the ball on to his off bail. It was the ninth time Pietersen has fallen to Siddle in Tests.

Pietersen had hit two fours and three sixes in his 90-ball 50, following Root to the milestone but at a slightly quicker tempo.

Alastair Cook's departure to Johnson for a single provided a deflating start to England's rearguard.

He mishooked to a tumbling Ryan Harris, and then 40 minutes later Michael Carberry got a little more bat on the same shot to Siddle but had to go too when he picked out Lyon - sliding on the boundary to take a smart low catch.

Root and Pietersen were therefore presented with an unenviable task, but one someone had to take on if only to salvage some pride and belief to take into the remainder of the series.

They did so admirably too until Pietersen was undone.

Shane Watson again played a part in his bowling partner's success, his miserly medium-pace stagnating the scoring rate as ever and Pietersen falling after three successive maidens.

Clarke then cheekily revisited the Smith factor for an over before tea, and it was too much for Ian Bell.

England's most accomplished batsman went up the wicket to meet the ball on the full but hit it low to mid on where a delighted Johnson stooped for a telling catch.

When Root then went too shortly after tea, it seemed England might be doomed to a second successive Test defeat inside four days.

Stokes and the badly out-of-form Prior refused to make things easy, though.

The debutant left-hander survived two DRS reviews, as Australia tried to win a Johnson lbw verdict and then one for Harris caught-behind.

Stokes was then given not out again by Kumar Dharmasena but would have gone lbw to Harris for 12, had the hosts retained a review before the 80-over mark.

It was a gritty two-hour innings from the all-rounder nonetheless, and Prior was steadfast too - if far from convincing - after his sequence of 0, 4 and 0 so far in this Ashes.

It was not until the second new ball was taken, and after England had topped 200 for the first time in the series, that Harris - previously wicketless - had Stokes fencing a catch to Clarke at second slip.

That meant Stuart Broad must walk into the firing line against Johnson, cranking up the pace under lights and heavy cloud cover.

But with some fortune on their side - and more late aggro from Johnson, who had previously made Stokes well aware of his thoughts - England had just enough resources left to push their hosts into a five-day contest.