If you'd been told before this series started that we would be heading to Perth on the back of two draws, you probably would have despaired. Yet the first two Tests have had everything from a day when the record books were rejigged by some Godlike batting to one where grit and determination were tested to the absolute lengths.

Both types of day have entertained magnificently, and it now seems quite appropriate that the two sides take the field in Perth, a venue known to produce results, with everything on the line. It all comes down to this, and so it is an apt scenario for a champion like Ricky Ponting to finally take his leave.

In Michael Clarke Australia have had the standout player of the series, yet there's no doubt who has shown the most mental strength. Indeed South Africa's mental fortitude did not draw the praise it deserved after Adelaide. Battered into submission on the opening day, the feeling was that the psychological bruises would be harder to dust off than the score itself.

Yet the Proteas slowly eked their way back into the game, and even after they slipped to the edge of the abyss again on the fourth day, they somehow had the strength to cling on and fight their way back again. Perth will show whether they have been pushed so far that they will finally crack, or whether that superior mental strength will win out as both sides feel the physical strain.

Indeed both teams have been forced into changes that are less than ideal - Australia because they have been forced to swap their entire seam attack; South Africa because their structure is so delicate that removing one key player from the equation upsets their balance.

In some respects that has made Australia's decisions easier to make, and the selectors have once again acted clinically. While Mitchell Johnson, Mitchell Starc and Josh Hazlewood represent an inferior attack to the one that played the first two Tests, the switch to a largely left-arm angle will force the South African batsmen to do more homework and rejig their techniques.

The return to Shane Watson will also be a huge boost, giving Australia a proper fifth bowler for the first time in the series as well as bolstering a top three that has failed to lay a platform for the middle order.

South Africa's equation is a tricky one to put together until they know the status of Jacques Kallis, who trained on the eve of the match and now looks likely to play as a specialist batsman. However that would set off a chain of events lower in the order to make up for the absence of a fifth bowler, and the exact composition can only be guessed at by all but the selectors.

The suspicion at the moment (and this could change depending on what the pitch looks like on Friday morning), is that Jacques Rudolph will lose his place to Robin Peterson, who will add depth to the tail and provide a holding spin option as Imran Tahir gets the chop and Vernon Philander returns. Were this the first Test of a series then the Proteas might be content to go in without the fifth bowler, but the workload over the first two matches means that they will probably be reluctant to do that here.

However should Kallis not make it, then Dean Elgar or Ryan McLaren would come in and provide an all-round option, and Rudolph would get another match to save his Test career. There are several variables, and the only certainty is that the on-tour selectors will try to provide as much continuity as they can under the circumstances.

Key Men
Australia: Michael Clarke's back-to-back double hundreds have given Australia the upper hand and covered up Ricky Ponting's failures, and it's difficult to see him getting out cheaply with the way he's playing. There's no doubt that he holds the key to the Aussie innings, so his wicket will once again be crucial.

South Africa: The Proteas have beaten the bat on numerous occasions during the series as the ball did too much to get rid of the likes of Clarke. But on a pitch that will probably provide just a touch of seam movement, Vernon Philander could be the man to get the right amount of nibble.

Last Five Head-To-Head Results
November 2012, Second Test: Match drawn at Adelaide
November 2012, First Test: Match drawn at Brisbane
November 2011, Second Test: Australia won by two wickets at the Wanderers
November 2011, First Test: South Africa won by eight wickets at Newlands
March 2009, Third Test: South Africa won by an innings and 20 runs at Newlands

Prediction
Australia have been the better side in the first two Tests, but haven't landed a knockout punch. Having been forced to change their entire seam attack, South Africa may well make them regret it.

Online betting firm Sky Bet has Australia as favourites in the betting for the second Test. Visit Sky Bet for the latest cricket betting.

Probable Teams
Australia: Ed Cowan, David Warner, Shane Watson, Ricky Ponting, Michael Clarke (capt), Michael Hussey, Matthew Wade (wk), Mitchell Johnson, Nathan Lyon, Mitchell Starc, Josh Hazlewood.

South Africa: Graeme Smith (capt), Alviro Petersen, Hashim Amla, Jacques Kallis, AB de Villiers (wk), Faf du Plessis, Robin Peterson, Vernon Philander, Dale Steyn, Rory Kleinveldt, Morne Morkel.

Dates: 30 November-4 December
Morning session: 10:30 - 12:30 (02:30 - 04:30 GMT)
Afternoon session: 13:10- 15:10 (05:10 - 07:10 GMT)
Evening session: 15:30 - 17:30 (07:30 - 09:30 GMT)
On-field umpires: Richard Kettleborough and Asad Rauf
Third umpire: Billy Bowden
Match referee: Ranjan Madugalle

Tristan Holme