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He ended an absorbing day two unbeaten on 112, in the midst of a 215-run partnership with hometown hero Henry Nicholls, who is battling a calf injury but survived to be 89. At 286-3, New Zealand are well on top as they look to sail past Pakistan's 297.
Williamson is a marvel, but let's park that there for a moment and focus instead on the fine margins that can separate success and failure – margins so fine they can be measured in millimetres.
Whether it's a cross-court forehand on match point that just misses the line, or a potentially game-winning three-pointer that catches too much of the rim, stories of joy turn to lament and vice versa.
If the New Zealand national anthem rings out around Lord's in June, a few millimetres might have made the difference.
Make no bones about it, New Zealand were in trouble here. They were 74-3 and trying to claw their way to parity when Nicholls hung the washing out to Shaheen Shah Afridi and Shan Masood gratefully accepted the chance at second slip. Nicholls turned on his heel and left, sensibly avoiding eye contact with his skipper who was battling his rig off on a ground that has never been kind to him.
The call came through from third umpire Wayne Knights. Shaheen had overstepped – no ball.
In that fine-margin moment the match changed and with it, quite possibly, the fate of New Zealand's test championship campaign.
Nicholls followed Williamson's lead and left when he could leave, defended under his eyes and scored when the opportunity presented itself. They had some luck, sure, but no more than Pakistan enjoyed on day one.
While the scores at the end of the day may look similar, the cricket was markedly different.
Whereas New Zealand went full, often too full, in the search for wickets on day one, conceding quick runs in the process, Pakistan were happy to stay back of a length.
It was hard to argue as they pegged New Zealand back after the loss of both openers.
The two-Toms opening partnership had looked La-Z-Boy comfortable against the new ball and then found themselves dismissed within minutes of bringing up a 50-run partnership.
Blundell (16) was the first to go, caught on the knee roll when only half forward. Faheem Ashraf's appeal was turned down by Chris Gaffaney but ball-tracking had it catching enough of leg stump to overturn.
Latham quickly followed as Shaheen Shah Afridi was brought quickly back into the attack to go at new man Williamson. Instead it was Latham (33), who poked at a ball and saw it dropped by Shan Masood – the start of an ugly theme – only for Haris Sohail to react brilliantly at first slip.
Ross Taylor came and went for 12 and in came Nicholls, in came Shaheen, in came the edge that could have changed the match… and in came the third umpire.
If that was unlucky for Pakistan – and even that is debatable as over-stepping remains within control of the bowler – then what followed was pure bad management.
Mohammad Rizwan has won a lot of admirers with his gutsy batting and considered leadership, but this was a day he got it badly wrong.
With half an hour to go to tea and Williamson and Nicholls struggling for any fluency, Rizwan didn't just take his foot off the throat but gave the batsmen a lozenge and a nice cup of tea to go with it.
Out of the attack went Shaheen, Faheem and Mohammad Abbas, their best operators, and on came part-timers Masood, Sohail and debutant spinner Zafar Gohar.
Batsmen of this class need no second invitation. Williamson in particular used the opportunity to move from second gear to overdrive, clipping and pulling through the leg side. By the time Rizwan went back to his front liners, Williamson and Nicholls were set.
They went to tea on 145-3 and Williamson was in a hurry after the resumption. In one over from teenager Naseem Shah he hit four boundaries, two clips through midwicket, a sumptuous on drive and a wild slash that burst through Masood's hands at second slip.
That took him from 78 to 94. A hook for four and leg glance to the boundary took him past 100 for the first time since the last test, which was the first time since the test before that. Yeah, he's good.
He should have been caught for 107 – yes, it was Masood again – and a quirk of cricket's rules saw him survive a close run out call on the same score.
Nicholls was dropped on 86, a sitter to Rizwan that summed up the skipper's day. If it feels like Nicholls has been overlooked a little in all this, then accept this as an apology.
Remember though, there are worse things in life to be than second fiddle to Williamson.
Tomorrow could be his day.