"We've still judging it day by day and it is improving each day," he said.
"The load on the thumb itself hasn't been great but it's okay. There's a bit more flexion each day.
"I guess it'll come down to the staff and myself talking and I guess with a lot of cricket coming up and how the thumb is going into the match, all those things have to be considered."
That latter is an important point. Williamson is an integral part of the squad.
The dilemma for coach Mike Hesson, the medical staff and the player himself is should he play on the basis that the hand feels "okay" while batting in the nets, but will need protection in the field, or should he sit out this test and avoid risking further damage, with four more coming up in the next 10 weeks.
"You want to be playing in a test for your country, and at home," Williamson said.
"I've never had an injury where I've been out and missed matches, so it's a bit of a change. We've had some good weather at the Mount [Maunganui] so I enjoyed a few days off but it didn't last long before I was itching to pick up a bat again."
Williamson is a regular, and impressive close catcher, often in the gully. That won't happen should he make the final XI. To place him there would be foolish. So a rejig is needed in the slip-gully cordon.
Aaron Redmond, five years after his last test, is Williamson's cover. His form is good and there's a solid case that, on his home ground, he deserves an opportunity if Williamson is still feeling the thumb by tonight.
West Indies management yesterday indicated the aggressive, bouncy Tino Best will lead their test attack.
With the word out on Williamson, the batsman knows he will have plenty of short bowling to contend with.
Best is no Dale Steyn or Mitchell Johnson, but he and either Sheldon Cottrell or Shannon Gabriel will be no different from other fast bowlers who sniff a chance to discomfort a tentative batsman.
So it comes down to risk and reward, and the risk of playing Williamson seems high. Equally it appears that course will most likely be taken.
Both teams have their final practice today.