According to a Japanese Seiken Densetsu guide book (specifically, the one with the blue bottom and "SQUARE BRAND" in the bottom right corner), one of the game's programmers claimed that it apparently began life as an experimental tennis game.

"This game actually started out as an experimental tennis game. At some point, the court turned into scrollable screens, the racket turned into the playable character, the ball turned into a weapon, the opponent’s racket turned into enemies, and the “court” became referred to as the “map”… And before I realized it, a story was added into it and then the game was released as “Seiken Densetsu.” It was a curious experience."