The Baby Blacks beat the Japanese under-20 team 38-21.
Unusually, the game was called off with 12 minutes remaining as the AIA Arena in Kutaisi was hit by a lightning storm.
The game was delayed by 20 minutes before the authorities decided the clash would be abandoned but the scoreline would remain.
That was a mixed blessing for the young New Zealanders.
The danger of lightning is real but the abandonment meant they potentially missed an opportunity to rattle on more tries, as they had all the momentum.
Southland and Highlanders first five Mika Muliaina played a big role in the game.
He scored a try, kicked four conversions and combined nicely with classy Bay of Plenty halfback Charlie Sinton.
The Baby Blacks jumped to an early 21-7 lead but Japan, seeking their first win at the tournament since 2015, levelled the scores by halftime.
That was a wake-up call for the New Zealanders, who regained control by playing some good wet-weather rugby.
They dominated in the set piece and scored three unanswered second-half tries before the game was abandoned.
Hooker Josh Findlay did a lot of grunt work around the park, and blindside flanker Bradley Tocker transitioned seamlessly from the All Blacks Sevens programme.
The Baby Blacks next play Scotland on Friday.
Junior World Championship
The scores
New Zealand 38
Dane Johnston, Bradley Tocker, Ollie Guerin, Logan Williams, Caleb Woodley, Mika Muliaina tries; Muliaina 4 con
Japan 21
Kise Shin, Shinnosuke Uchida, Sota Miura tries; Takeru Niwa 3 con
Halftime: 21-21.
