Clearly she isn't planning to use a swimsuit to charm Kei. I guess for an ethusiastic athlete, using something related to sports for unrelated things would be a sacrilege. Or it was so mundane for her that it never visited her mind.
ok, so what I think got lost in translation: sometimes girls refer to their sexy underwear as their "battle underwear", when she said "for contests" she might have worded it in a way the mayu misunderstood as that.