Steven Smith has raised the bat for so many centuries that it seems trifling for him to celebrate a 50. But this week at the SCG he will do exactly that when he walks out for his 50th Test match. If it is not a particularly rare milestone - 50 Australians before Smith have reached the 50-Test mark - it is at least an opportunity to reflect. And for Smith, the reflections are startling in their contrast.

It was against Pakistan at Lord's that Smith made his Test debut in 2010, batting at No.8 behind fellow debutant Tim Paine. In hindsight that seems remarkable - Paine still has only one first-class century even now, compared to Smith's 29. And even at the time, Smith's batting potential was obvious. In the previous summer he had made four Sheffield Shield centuries, more than any other player in the competition.

But Smith was picked as a legspinner for his Test debut and the sometime rigidity of Australia's selection habits - Cameron White had also batted at No.8 when picked as a Test legspinner two years earlier - meant that he batted below the wicketkeeper. Six years later, Smith will enter his 50th Test match as the No.1-ranked Test batsman in the world, and with the phenomenal Test batting average of 60.63.

That means that among batsmen with at least 4000 Test runs, Smith sits third on the batting averages behind only Don Bradman and Herbert Sutcliffe, an unbelievable achievement for a man first picked as a bowler. And with 4669 runs from 90 Test innings, Smith is very well positioned to become just the third Australian after Bradman and Matthew Hayden to take less than 100 innings to reach the milestone of 5000 Test runs.

"I was fortunate to play when I was quite young, obviously as a spinner when I debuted in England," Smith said in Sydney on Monday. "I've always seen myself as more of a batsman and that's probably one of the best decisions I've ever made, to put that [bowling] on the backburner and just concentrate on my batting. It's been a good couple of years, I've enjoyed every moment of it and playing my 50th Test now - it's an exciting time and hopefully I can have some success this week."

Not that Smith was not an accomplished batsman when he earned his baggy green - as his figures from that 2009-10 Sheffield Shield summer prove. In fact, when he made his Test debut, he was already averaging 56.22 as a first-class batsman. But much honing of his technique was required in order to progress from a natural talent who relied on a good eye to the consistent high-class performer on display today.

"Probably walking about a foot across my stumps," Smith said when asked what he would identify as the biggest difference between his batting in 2010 and 2016-17. "I used to bat on middle stump and not have any prelim movements. That was actually something I started in the 2013 Ashes out in the middle of the WACA. They were bowling quite short at me out there and I started a prelim movement, it was like everything sort of just clicked into place.

Steven Smith made his second Test century Getty Images

"It's gradually got a little bit bigger over time, but it's just getting me into a position to pounce on anything. Whether it's full or short - I know where my stumps are and I get people bowling at my stumps, which is nice. I think that was a big turnaround for me, and everything has felt pretty good since then. So hopefully that can continue."

The Perth Test of which Smith spoke came less than a year after his second coming as a Test cricketer. When he was picked in a 17-man squad for the tour of India in early 2013, Smith appeared the 17th-most likely to play a Test. The selectors made it clear that Smith was a backup batsman rather than an allrounder, but the structure of the side meant that he wasn't even the first backup: that was clearly Usman Khawaja.

But then came the homework saga, and it is arguable that no Australian benefited more from that fiasco than Smith. The likely scenario after Australia's loss in the second Test in Hyderabad was that the struggling Phillip Hughes would have been dropped and replaced by Khawaja. Instead, Khawaja and Shane Watson were suspended for not doing their homework, so Hughes was retained and Smith came in for his first Test in more than two years.

He quickly proved what he had learnt in that time away from the game. In the first innings in Mohali, Smith struck an assured 92, and since that moment he has not missed a single Test. In fact, since his Ashes century in Perth, the second of Smith's Test career and his self-described turning point, nobody in the world has scored more Test runs than Smith's 3844, which he has made at the remarkable average of 73.92.

Second on the list of Test run-scorers in that same period is Joe Root, and in third position is Smith's vice-captain, David Warner, who has 3407 at 53.23. At the MCG last week, both Warner and Smith scored the 17th Test century of their respective careers, and they are without question the two key planks of Australia's batting order. They are also well aware of their numbers, and enjoy the challenge of trying to outscore each other.

"There is that sort of healthy competitive nature there," Smith said. "I know after he got his hundred in Melbourne, I said 'oh I'm going to have to get one now'. It's obviously not that easy, but fortunately I was able to equal him and try and keep it going for this Test match. I'm sure he'll be keen to go out there and go one better. I'm going to have to do the same."

Perhaps only Warner can match Smith for the most surprising Test career trajectory in Australia's current line-up, the Twenty20 slogger who turned into one of Test cricket's most productive batsmen. Like Warner, Smith emerged during the T20 era, and like Warner, he has mastered the long format.

"I always wanted to play Test cricket," Smith said. "That was the pinnacle for me and I just love batting, and love batting for long periods of time. You don't get to do that in T20 cricket, so I love playing Test cricket."

The Australians love having him. Perhaps the scariest prospect for opposition teams is that Australia could have Smith for another decade, for he is still only 27. For now, it might only be a half-century that he raises when he walks onto the SCG on Tuesday morning, but what an extraordinary 50 it has been.