Watling batted throughout the third day and was 119 not out at stumps when New Zealand was 394-6 in reply to England's first innings of 353. Santner was 31 and had shared a 78-run partnership with Watling for the seventh wicket.
Previously, Watling put on 70 for the fifth wicket with Nicholls (41) and 119 for the sixth with de Grandhomme (65) to give New Zealand a first-innings lead after it began the third day on the back foot at 144-4.
Watling's resilient innings was exactly what New Zealand needed to reach a lead from that position. He reached his century in just under six hours, from 281 balls and by stumps he had occupied the crease for 418 minutes.
Watling's innings ensured New Zealand, for the first time in the match, ended a day's play with a convincing advantage. The first day belonged to England which was 241-4 after winning the toss, and the initiative passed freely between the teams on the second day before England was dismissed for 353 and New Zealand finished four down and more than 200 runs behind.
New Zealand had briefly played itself into a strong position on the second morning when seamer Tim Southee took three quick wickets, including the key wicket of Ben Stokes for 91, to reduce England from to 295-8. But a 52-run partnership for the ninth wicket between Jos Buttler and Jack Leach put England back in charge.
New Zealand quickly lost its openers when it replied but captain Kane Williamson had looked impregnable reaching 51 before he fell just before stumps on Friday. His dismissal again tilted the match in England's favor.
Watling's partnership with Nicholls before the end of play on day two and lunch on day three revived New Zealand's fortunes. Nicholls was struck on the helmet by England speedster Jofra Archer just before the end of play Friday and needed to pass a concussion protocol test before resuming his innings Saturday.
Nicholls batted for more than 2 1-2 hours to steady New Zealand's innings. Watling and de Grandhomme then produced the century partnership for the sixth wicket which helped New Zealand creep past England's first-innings total.
"To see B.J. bat throughout the day and then the contributions throughout the day from Colin and also Mitch to still be going, it was great," Nicholls said. "We knew what to expect from the England attack, they showed that last night, so it was good for us to get through that and for B.J. to anchor the whole day's play."
De Grandhomme batted with discipline and fortitude to reach a half century from 73 balls and to help tip the balance of the match in New Zealand's favor. Watling and de Grandhomme carried New Zealand to 316-6 at tea, in sight of England's total. But in another of the twists typical of the match, de Grandhomme fell to the first ball after tea and the game was again in balance.
While he batted with great concentration and stamina, Watling also lived a charmed life. He was dropped by Stokes at first slip when he was 31 and he survived three lbw appeals with the help of the television umpire.
England's bowlers worked hard throughout the day with little luck. Stokes perhaps was the best, troubling the batsmen with bounce and finishing the day with 2-37 from 16 overs. England coach Chris Silverwood said he was pleased with his team's performance Saturday.
"The thing I enjoyed is that we gave time for the plans for work," Silverwood told Britain's Press Association. "I thought we showed a great attitude and put a lot of effort into it. It's been a good hard-fought day of test cricket."
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