The Australians are catching everything in sight, their bowling is incisive and they are scrambling enough runs when they need them. They are tactically innovative – witness the dismissal of Graeme Smith at a cannily poised short backward square leg in the second innings at Centurion – and they are playing with confidence. Australia, suddenly, is achieving feats beyond the sum of its individual talents, less than a year after presenting in India as a directionless rabble.

Momentum is a mighty force to counter in tightly-scheduled Test series, so Australia is favoured to win the second Test, and hence their series against the incumbent world no.1 South Africa.

But Test cricket supremacy is more of a matter of opinion than in most sports. The official ICC Test rankings are complicated. Without a Test World Championship, assessing the comparative worth of teams is difficult. They play different amounts of games against different opponents at different times of the year. It may be that the inability of cricket to definitively assess its Test-playing nations has sustained the 50-overs World Cup. Limited-overs cricket can be handled by a regular tournament. The five-day format defies such neat devices.

The criteria behind the oft-ignored Test ratings are worthy. Performances are rated over a four-year period, with more recent series given twice as much weight as older performances. The rankings reward wins away from home, and against stronger opponents. A team has to play well over a stretch to gain top billing.

At the outset of the current series, South Africa was rated the dominant Test team, at 133. India was second at 117, Australia third at 111. After its shock first Test capitulation, South Africa is 127, Australia 115. But a 3-0 win to the visitors would only narrow the gap: 124 to 117.

The Proteas deserve to maintain a high rating. They last lost a series in 2008-2009 (to Australia at home). In its past 31 matches, it has won 20, drawn seven and lost just four. Before its recent purple patch, Australia lost seven Tests out of eight, and went nearly a year without a win. In its past 32 Tests, Australia won 17 Tests, drew five, but lost 10 times. It won only four times off Australian soil, two of those wins coming against the beleaguered West Indies in the Caribbean.

The ICC guidelines state the following: "A rating above 120 suggests consistently strong performances. Above 130 is rarely achieved and suggests a high degree of dominance over all other teams."

In building such a record, South Africa has been more than a bullying front-runner. Against India in its most recent series, it made 7-450 in the fourth innings, chasing 457, to draw. It then won the second Test comfortably to take the series. Prior to that, the Proteas drew with a resurgent Pakistan in the Middle East after losing the opening Test of the series, fighting back with a completely dominant second Test, winning by an innings and 92 runs.

The Proteas again overcame a sluggish start to defeat Australia in late 2012, surviving 148 second innings overs in Adelaide before pummelling a weakened Aussie bowling line-up in Perth.

These are the performances of a team with depth, experience and self-belief. South Africa has a record of sustained excellence, playing at home and away.

But the Test rankings, however worthy, are achieved by obscure maths. The evidence of one's own eyes is always more compelling.

It may too early to declare Australia the world's best team, but a win in the Second Test would be definitive. Series wins at home, comprehensively, and away to the Proteas would provide a strong case that the rankings are lagging behind a newly dynamic paradigm, where the balance of power between the Test nations can shift dramatically with minor changes in personnel.

Pakistan and now New Zealand are starting to re-emerge as competitive Test teams. India is formidable on home soil. It could be that we are about to enter a period in which overall supremacy will be more keenly contested than in decades, no single power as dominant.

The first Test of the current series felt a lot like the seismic shift experienced during the recently completed Ashes marathon. Australia's previously dominant foe, supposedly superior, being shocked by a happy, keenly focused team effort, and totally overwhelmed by its revitalised quickie Johnson.

England's 3-0 home victory in 2013 over the Aussies proved a little flattering to the hosts, unprepared for the intensity of the assault unleashed on Australian soil. South Africa's previous series against Australia had been very tight as well. The Proteas have made a habit of overcoming slow starts to series, but they looked sluggish and shell-shocked at times during the First Test.

The gap between Test teams is not as great as the rankings, and individual averages indicate, and the addition of a couple of powerful forces on one side, and their subtraction on the other side, is enough to tip the balance.

England eventually lost a batting linchpin Jonathan Trott, and the cornerstone of its bowling attack, offie Graeme Swann, by the end of three Tests in Australia. It became apparent an era had swiftly come to an end before our eyes.

It is tempting to think the same of South Africa, but surely it would be premature.

In that series-losing Test in Perth in late November 2012, Johnson was a second-string replacement for the sidelined Peter Siddle, Ben Hilfenhaus and James Pattinson. Injury subtracted Ryan Harris from the entire series.

Johnson in his present form is a completely different creature to the 2012 model which leaked 4.40 runs per over in the second innings as Graeme Smith, Hashim Amla and AB de Villiers walloped the Australians out of the match and the series at the WACA.

And Ryan Harris has claims to be considered the equal of any other fast-bowler in the world. They are two huge changes, enabling Australia, with the continued consistency of the redoubtable Peter Siddle, to pressure a batting line-up throughout an innings.

The glaring change on the Proteas side of the ledger is the absence of Jacques Kallis.

He did not leave international cricket with his powers much diminished – a match-defining century in his final match set the Test win up for South Africa. His stints at the bowling crease eased the load on the pacemen, and invariably yielded wickets. There is an argument that the impact of the quietly spoken all-rounder was underrated. It is unlikely to be so in his retirement.

When South Africa last beat Australia, Faf du Plessis, fresh off a monumental, match-saving debut hundred, batted at seven.

He now bats four, a key plank of the Proteas efforts to combat Australia's ebullient bowling attack.

With Johnson the most potent force in world cricket, Siddle and Harris as formidable as Morne Morkel and Vernon Philander, and Nathan Lyon clearly a superior spinner to Robin Peterson, Australia has the edge in its bowling.

South Africa possesses more proven batsmen.

Right this minute, the Proteas deserve to retain their status as the best Test team in world cricket.

But they must find a way to survive Mitchell Johnson. Rarely has one player so single-handedly changed a Test team.

If they don't, the crown may slip by this time next week.