The third quarter was scoreless, although Merry came desperately close. After Curtis got an initial touch in front of the Japan goal, it deflected high to Merry, whose chest high reflex job bounced off the cross bar.
The deadlock was broken a minute into the fourth quarter when Merry got a deserved reward, jabbing the ball into the Japanese net from close range after a long ball from Brooke Neal was deflected into her path by Pippa Hayward.
The loss of captain Anita Punt to a yellow card - one of three in the game to go with four green cards - was then immediately followed by an unexpected Japanese equaliser. They broke down the right through their best attacker, Mie Nakashima, and New Zealand keeper Sally Rutherford, believing a cross was coming was well off her line and caught shot when Akiko Kato drove the ball into the goal.
However 10 minutes from the end, New Zealand were back in front from a penalty corner when Ella Gunson's nudge in a hectic scramble in front of Japan's goal found its way into the net.
While Canterbury attacker Merry was the pick of the New Zealand side, there were strong contributions from attackers Stacey Michelsen, Cocks and Curtis, while Liz Thompson, Hayward and Aniwaka Roberts defended industriously.
"I thought we played pretty well for the majority of the game, overall I was really pleased," New Zealand coach Mark Hager said.
''To win a penalty corner and put it away was good, I'm just happy to get the three points.
"It's disappointing that we let Japan equalise, we just need to ensure that we play well for the whole game. We will get punished if we have any defensive lapses."
World No 1 Netherlands beat China 2-0 in the opening game, and next play the Black Sticks in a crunch game in pool A early tomorrow.
Australia were playing England and hosts Argentina faced Germany in pool B games later in the morning.
