Hinata Miyazawa scored in the 43rd and 62nd minutes while Mina Tanaka, twice denied by the flag, netted in the 55th.
The magnificent Jun Endo scored a fourth in the 71st minute at Waikato's FMG Stadium.
Things got worse for Zambia when goalkeeper Catherine Musonda gave away a penalty, which Riko Ueki scored on her second attempt, and received a second yellow card, which will rule her out of their next clash with Spain.
Japan had already lost first-choice goalkeeper Hazel Nali to an anterior cruciate ligament injury before the tournament.
The victory puts Japan top of Group C ahead of Spain, who beat Costa Rica 3-0 on Friday.
Japan were simply electric at times with their ball movement and skilful crosses into the box, while keeping Zambia star Barbra Banda quiet.
In the 21st minute, Tanaka bundled home but was ruled offside after a VAR check.
Japan took the lead 22 minutes later when Fujino slipped through Yui Hasegawa, who took a lovely touch and teed up Miyazawa.
In the 52nd minute, Musonda upended Fujino and the Japan forward immediately received a penalty, with the goalkeeper booked.
But after a VAR review, Fujino was found to be offside in the build-up.
Three minutes later, Endo beat the offside trap and squared the ball for Tanaka, who deftly turned Japan's second goal home.
Seven minutes later, Tanaka turned provider.
Tanaka showed some brilliant desperation to keep the ball in and cut it back for Miyazawa to bury Japan's third.
Japan added a fourth after a wonderful build-up, which finished with Hasegawa splitting Zambia's lines with through ball to Endo, who coolly burst forward and scored.
Zambia's night went from bad to worse in the 97th minute when Musonda brought down Ueki, receiving a second yellow.
Replacement goalkeeper Eunice Sakala tipped Ueki's first penalty onto the bar but was ruled to have encroached.
Ueki made no mistake with her second attempt.
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