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In a demonstration of the value of quality over quantity, the New Zealand skipper has used the four opportunities he's had at the crease in this otherwise dire year to compile 472 at an average of 118.
He might get one more chance to bat before the boom is lowered on 2020, but he'll be hoping it's a short one as New Zealand chase victory against Pakistan.
At the risk of using sentences that could be cut and paste from any number of stories, Williamson's classy century has put New Zealand in prime position to continue their home test dominance.
Whether that happens or not is up to the bowlers, who have yet to be found wanting this summer. This pitch doesn't look as complicated to bat on as the ones we have seen at Hamilton and Wellington so sweat might be required just as much as skill.
At the end of day two Pakistan were 30-1 from 20 overs and looked prepared to scrap as they started their pursuit of New Zealand's imposing 431. The wicket to fall was Shan Masood, who got the faintest of edges down the leg side to Kyle Jamieson (1-5), who was the pick of the bowlers.
Abid Ali was not out 19, with nightwatchman Mohammad Abbas yet to score as the day finished under the Bay Oval lights.
Whether New Zealand will have full use of Neil Wagner will depend on the results of scans he underwent last night. Wagner was struck flush on the boot by Shaheen Shah Afridi while batting and was in a fair bit of pain between innings. He was given pain relief and did bowl three overs late in the day but did not look at his sharpest.
Williamson, as is his wont, showed how it could be done. The batting savant started the second day on 94 and eased his way to a 23rd test century, increasing his lead over Ross Taylor, who scored 70 on day one, by four.
In many ways it mirrored his 22nd century in that he reached stumps on day one within a single shot of his milestone having spent the majority of the day in a battle of attrition with the bowlers.
If anything, the struggle here was greater than it was against the West Indies in Hamilton because the attack led by 20-year-old left-armer Shaheen (4-109) was more skilful and relentless.
Williamson only looked to be batting with relative freedom once he passed three figures and it was a surprise when he pushed hard at one from Yasir Shah and was taken low down at slip by Haris Sohail (nobody was quite sure what the umpires saw to give the soft signal of not out before that was overturned).
His 129 moves him alongside Kevin Pietersen, Virender Sehwag, Justin Langer and Javed Miandad on the all-time centuries list. Those four all played more than 100 tests; Williamson is playing his 82nd.
It wasn't a one-man show by any stretch. Taylor was excellent, BJ Watling punched and clipped his way to 73 and Henry Nicholls looked largely untroubled in getting to 56 before neglecting to review his dismissal caught off a bouncer when replays indicated he neither nicked nor gloved it.