Williamson passed The Don, aka Sir Donald Bradman, en route to 7000 runs, while Nicholls made his century. Their fourth-wicket partnership is a record-breaker and the team approached the 400-run mark for the innings as they took a lead early on day three.
When the pair reached 343 they secured New Zealand's all-time highest partnership for the fourth wicket, beating the 271 set by Ross Taylor and Jesse Ryder against India in 2009.
Pakistan played their part, too, continuing a horror innings in the field by dropping Nicholls on 92.
Earlier, Nicholls' participation was in doubt, having strained his troublesome left calf muscle yesterday. He had a net session this morning to determine whether he could continue.
New Zealand are seeking a big first innings lead in their push for victory as a win would give them a decent chance – depending on results – of making the World Test Championship final scheduled for Lord's in June.
Williamson and Nicholls, who came together when New Zealand were in trouble at 71-3 chasing Pakistan's first innings 297, are playing their part and rolling past a number of significant milestones along the way.
Yesterday Williamson brought up his 24th test century, increasing his New Zealand "lead" over Ross Taylor by five, and joining Sir Viv Richards, Greg Chappell, Mohammad Yousuf and David Warner, all of whom have taken more innings to reach that number.
It turns out he was only getting warmed up.
At 120 he went past the mythical 6996 test runs scored by Sir Donald. There are standards set by Bradman that will never be surpassed, such as his 99.94 average and his scoring a century 36.2 per cent of the time he strode to the crease, but it's nice to give a nod to the greatest as you pass his aggregate.
Achieving it in his 144th innings means only 12 batsmen in the history of the game did it quicker, a who's who of 11 of the greatest to have ever graced the crease, and Matthew Hayden.
At the other end was Henry Nicholls, who started the second test against the West Indies under a scintilla of pressure to hold his place after some indifferent form. He scored a test-best 174 in that match and followed it up with a half century last week at Mt Maunganui.
Here he started the day on 89 and has just posted his seventh test century, joining Glenn Turner, Andrew Jones and Bevan Congdon, achieving the mark with a powerful cover drive.
On day one he became the 29th New Zealander to score 2000 test runs.
