半数のプロテニス選手は収入ゼロ
http://edition.cnn.com/2016/10/18/tennis/tennis-academies-simona-halep-madison-keys/index.html
98%のプロテニス選手は賞金では赤字
95%のプロテニス選手はスポンサー収入含めても赤字
http://www.dailymail.co.uk/news/article-2916186/Only-one-FIFTY-tennis-players-afford-cover-160-000-costs-year-involved-competing-circuit.html