Wales leaked two tries in the first spell with Kilryong So and Kenji Shomen darting over for the Asian side with Wales's only reply coming via Rhys Shellard who caught Japan on the hop when he scored in the first minute.
Staring a shock defeat in the face, the Welsh defence tightened up in the second half and squeezed the life out of Japan before Cuthbert raced over in the third minute to level matters. Aaron Bramwell edged Wales in front with the conversion and Paul John's men were able to withstand the Japanese onslaught to reach day two unbeaten.
All eyes will now be on the remaining two pool matches tomorrow as only the two best runners-up qualify for the Cup quarter finals.
England, champions of 2002 and 2006, got their challenge off to a flying start against Hong Kong with a 45-0 rout so Wales will have to match that avalanche of points and beat England to guarantee safe passage through to the knockout stages on Sunday.