The Swiss pro could have secured qualification to the ATP World Tour Finals in London with a run to the final, but it wasn't to be in Tokyo.
Tatsuma found the breakthrough in the opening set by breaking the second service game of Wawrinka, who eventually recovered with a break of his own to draw 4-4 on games.
However, the Australian Open champion was broken again, which allowed Tatsuma to serve for the set and take the lead on the hard court.
Wawrinka was dealt an early blow in the second set as he lost his opening service game, and later squandered two break points of his own.
Tatsuma secured another break point, which was all that was needed for him to wrap up the contest.