The Japanese scored first through Kilryong So but the Welsh struck either side of half-time, first through Ifan Evans and then James Lewis. A new look Wales side attempted to spread the ball but the slippery conditions made free-running rugby a lottery.
With 30 points on offer to the Cup winners in Hong Kong, the 24-team event could go a long way to deciding the outcome of the Series.
With only one side guaranteed a place in the Cup quarter finals from each pool, the onus is on every team to score as many points as possible in Hong Kong.
That means Wales must go out first thing tomorrow and rack up the points up against China in their second pool game before facing the in-form English. China were walloped 54-0 today by England and Paul John's side are fully aware the red rose army pose a major obstacle if they are to progress to the knock-out stages.