Kane Williamson had just reached fifty. Matters were somewhat stable though in no way were New Zealand out of danger. They were only 34 ahead and four down. More pertinently, Yasir Shah was in the middle of a tidy little spell, buoyed no doubt by the breaking of a record. We were in the 38th over of the innings and lunch was a couple of overs away.

Yasir found some drift on this one, a little like the drift he got in that spell in Dubai, like somebody holding the planet tilts it one way and suddenly everything kind of slides to that side. It was not a good length but a great length. Williamson went back to it, which looked for all the world like a mistake. In truth any movement looked a mistake.

The ball fell somewhere between middle and leg and Williamson, no doubt having read it was the leggie, knew what would happen next. Twice in this series, near-identical deliveries had done for him. In Abu Dhabi in the first Test, where he was bowled, the line was more middle and off. In Dubai, where he was caught behind, it had pitched outside leg and caught his outside edge.

Both those times it turned more, but this turned too and it was flatter and quicker. Back already, Williamson brought his bat down and with the outside half of his bat, bunted it to safety. He didn't bring it down in a hurry or in panic. It came down straight. His feet and body remained in fair shape. You can never know for sure but if you figured only a handful of batsmen would survive such a delivery, at such a moment, against such a bowler, nobody would say you are mad. None would have done it with as little fuss or made such a delivery look so not special.

Earlier in the session, just before drinks, Yasir had even bowled a googly at him. That doesn't happen often. And that too had spun and then kept low. That too Williamson had played out, in shape and without crumbling in a heap as so many batsmen do when a ball keeps low.

****

One quality of Kane Williamson's batting is that it can easily pass you by. With at least two of his three great contemporaries - and you would call them rivals if it was anyone other than Williamson we were talking about, a man who'd struggle to create a rival in a two-man shootout - you cannot help but know they're batting.

Virat Kohli at the crease, such is the energy, brings the challenges that gravity does to the watcher. Can't escape it. Steven Smith cuts such an odd batting figure that even if you've seen each of his 241 international innings, every time he starts it looks like something you've never seen before. Only Joe Root has this capacity to go unnoticed but because he is Joe Root, Boy Wonder now Captain of England, nothing he does, not even exhale, goes unnoticed. Plus the crowd won't ever let you forget it's "Rooooooot!" in action.

It's not at all that Williamson is an unattractive player. Au contraire. But he is so proficient with every shot he does play that his finest innings not only gather a sense of invincibility, they acquire the unfortunate byproduct of inevitability. Of course he played out those two Yasir deliveries. Of course he's going to rise up and punch through the covers off the back foot. Of course he's going to lean forward into those drives. Of course he combines moving forward with playing the shot late.

Which is crazy because the situation and circumstances of this innings had naturally dramatic ingredients. Nothing should have been inevitable about it. Yet you'd forget this was the third innings of a slow-scoring game, on a sluggish fourth-day surface given to inconsistent bounce, turn of varying pace available from day one, not just against a strong attack but one led by a world-beating leggie. Some broke sharp and bounced, others turned slower but also didn't climb as high. Runs have not come fluently for anyone, apart from Williamson that is. But every time Williamson was on strike the only conclusion was that yeah, this is what batting is supposed to be and Williamson is a batsman, ergo why are we getting our undergarments in such a twist?

Here's a theory: it could be about ownership, the one shot most great batsman turn into theirs and theirs alone, the one people wait to see and then store away in their minds, the one opposition bowlers try to first turn into a weakness and then leave well alone. Does Williamson have one? The back-foot cover punch could be, never an easy shot least of all on this kind of wicket. He played it for fun in this innings, and repeatedly against the turn, timing it like a Seinfeld punchline.

But it is an orthodox shot and he plays it without any unnecessary flourish. He plays it just as it should be played, as it has been played, and probably as it is written it should be played in some tattered old textbook. Plus Joe Root plays it as pretty as well. You could then pick another, but the truth is you could pick every shot he plays and the problem with that truth is that it leaves no particular one standing out.

An outrageous celebration could do, some manic bat-twirling or beating of the chest, some swearing, or a photogenic leap. A bat drop? No sir. When he reached his hundred - another back-foot punch through the covers getting him there, what do you know? - he waited a little before taking his helmet off. He looked towards the dressing room. And that was it. If there is an especially energetic or expressive Williamson celebration that is being missed here, tweet it in.

Kane Williamson and Henry Nicholls congratulate each other at stumps AFP

His metrics speak the same language his batting does. They sometimes pass you by, to the degree that remembering to include him in the big four is like a little win.

Neither his home nor away average is the highest among the quartet, so 55.06 at home won't leap out at you like Kohli's 64.68 or Smith's 77.25. And 49.93 away is just marginally less than Smith's 50.96, even though it's comfortably above Root (44.95) and Kohli (46.79). The difference between the two, though, is the smallest among the four. As is the difference between his first and second-innings averages - 52.60 and 51.18; Kohli averages 61.39 and 42.93, Root 55.75 and 43.53 and Smith 77.92 and 40.77. Even in this coding, Williamson is equipoised come rain, hail, hell or high water.

If you really wanted to stand him out from the quartet, you could throw his fourth-innings average at it. At 60.81, it towers above Root's (34.94) and Smith's (31.11) and at nine runs, is a low-rise building better than Kohli's. His average in Asia is better than those of Smith and Root and in wins, it is second only to Smith. Smith is the only other from the group to have averaged 60-plus in four of the last five calendar years.

Equally it's not difficult to let some humanness in through the cracks. He doesn't have a hundred in South Africa (but only four Tests there because, New Zealand) and averages less than 40 there and in England and India.

But at some point tomorrow a question could creep up on us. How good, maybe how great, was this innings? The result will colour assessments but it'll be worth reminding ourselves of the basic sternness of the challenge laid out in front of him - the surface, the trouble New Zealand were in, Yasir Shah and company (and usually, 'and company' doesn't even matter in these situations). Keep those two balls that he played for no runs, the googly that didn't bounce and the perfect legbreak, in your mind. No doubt some will remember a boundary here and there and some grump will of course point to the two missed chances, like it takes away from the innings.

Then you'll think back through his other hundreds and maybe the early match-saving hundred against South Africa will stand out. Or that eye-popping 140 at the Gabba, which, taken together, tell you something about the vast spectrum within which his batting can operate.

But as you go through the 19 you'll realise that so many of them have been of such a quality, of making difficult situations appear - as his batting coach Craig McMillan would later say - "ridiculously easy", that this one blends in with the others. Like his shot-making, if they're all so special then it doesn't mean none are special. It means that we need to redefine special.