American Johnson will become the first player to win all four World Golf Championships events should he defeat young Spaniard Jon Rahm later on Sunday at Austin Country Club.
Johnson needed a birdie on the 17th hole and a par on the last to defeat the 54th-ranked Tanihara in their semi-final.
The American had led 3-up after seven holes but Tanihara made four birdies in the next seven holes to level the match.
Rahm made the final with a 3&2 victory over American Bill Haas. The 21st-ranked Rahm closed out the match with three birdies in the last four holes they played. (Reporting by Gene Cherry in Salvo, North Carolina; Editing by Ken Ferris)