Amla and de Villiers set target of 632
Australia 163 and 0 for 40 need another 592 runs to beat South Africa 225 and 569 (Amla 196, de Villiers 169, Starc 6-154)
Scorecard and ball-by-ball details
Two days remain in this match, and two possibilities. The first, by far the most likely, is a convincing South African victory, set-up by the sparkling innings of Hashim Amla and AB de Villiers and finished off by Dale Steyn, Vernon Philander and Morne Morkel.
The second, still too outlandish to contemplate, is Australia finding a way to bat for long enough to reach a winning target of 632. David Warner and Ed Cowan made a sound enough start on the third evening to reach 0 for 40 by the close, but in terms of the task ahead they have barely reached base camp at the foot of Everest.
Amla and de Villiers provided a rich day's batting entertainment at the WACA ground, both falling short of double centuries but delighting spectators with their marriage of aggression and invention. De Villiers' century was particularly notable as his first while also carrying the wicketkeeper's gloves, opening the path to an extended stint in the dual role.
Australia's bowling was made to look ordinary in the extreme at times before Mitchell Starc and Mitchell Johnson found some heat in the early evening to polish off the South African tail. In all the visitors' innings lasted just 111.5 overs, the runs arriving at more than five per six balls, and its speed has left plenty of time for Australia's batsmen to negotiate. The pitch is still playing very well and the outfield extremely fast, but the visiting bowlers are refreshed and focused on the goal of wrapping up the match, the series and the ICC's No. 1 ranking.
Cowan and Warner began the pursuit with unhappy first innings memories; Cowan's of a golden duck, Warner's an unwise waft at Steyn that pinpointed the start of Australia's sharp second day slide from a promising position to a dire one. They fought out the closing overs in characteristic fashion, Warner hitting boundaries both certain and uncertain, Cowan watching the ball intently and surviving an exquisitely probing first spell by Philander.
There was some tension evident on South Africa's side when Cowan twice pulled away from the bowler, citing flies around his helmet. He exchanged words with the visiting captain Graeme Smith before a can of Aeroguard was called for. So far the Australian openers' stand has been merely a nuisance, but their survival placed a slightly different slant on a day that had been a South African waltz for most of its duration.
Resuming with a lead of 292, South Africa rose to salute Amla when he flicked Johnson to fine leg for his 18th Test century, a stroke representative of his legside mastery. Amla's movement across outside off stump to play to leg was exaggerated at times, but apart from the sliced drive from Johnsons' wide ball that just eluded Michael Hussey's reach he was seldom troubled.
Kallis rumbled along comfortably enough himself until Michael Clarke swung Starc around to the Prindiville Stand end, teasing out a top-edged hook shot that Johnson held well at fine leg, the ball dying into the breeze late in its path. De Villiers took his time to get established but accompanied Amla to the interval with an enormous amount of time left to stretch Australia's eventual target.
There was acceleration on resumption, de Villiers advancing to ping Nathan Lyon down the ground for a straight six, while Amla took advantage of Hussey's introduction with a pair of boundaries. Clarke responded to the calls of the WACA crowd by handing Ricky Ponting a bowl for the final over before the second new ball became due.
Given how Starc and John Hastings started with it Clarke might have been better off keeping Ponting on, as de Viliers and Amla attacked with impunity. It took Johnson's introduction to draw a few false strokes, and ultimately a wicket when Amla blocked a drive back to see the chance snaffled by Johnson's outstretched right hand.
Dean Elgar wore a sharp blow on the elbow before playing inside a fuller delivery to be pinned LBW - though this did not stop him from seeking an imprudent review in the manner of Ponting the day before. Two wickets in an over did little to unsettle de Villiers though, and the three reverse-swept boundaries to go to three figures summed up the marriage of skill and invention he has used so well as a batsman and now a wicketkeeper also.
After tea the runs arrived in a torrent, de Villiers taking progressively more liberties and Faf du Plessis showing the penchant for shots as well as forward defensives. Their concentration was broken when Clarke called for a highly speculative LBW referral against de Villiers, the break in rhythm proving more helpful than the video evidence. Johnson broke the stand next ball with a swift delivery going across du Plessis, and the final five wickets melted away for 31 runs. Whether that is of any consequence for South Africa will not be known until tomorrow.
Daniel Brettig is an assistant editor at ESPNcricinfo. He tweets here