How do triathletes and tennis players work on the ankle flexibility?

While I keep working on increasing my ankle flexibility, I recently read in an article that ankle flexibility comes at the cost of ankle instability (= higher propensity for ankle sprain), which spells trouble for runners and tennis players. Tennis being my other favorite sport, this puts me in a dilemma. Are there master swimmers out there who also maintain competitiveness in tennis or running, or is this a "you can't have a cake and eat it too" situation? I am hoping that I can become competitive in swimming with ankles that are stable (read rigid in swimming) enough for tennis as well.