Most Oaxacans are working too hard to be playing much tennis and golf. Consequently, tennis and golf courses are nearly completely restricted to Oaxaca City [1] and the Bahías de Huatulco [2]. If you are planning on a lot of golf and tennis, check into one of the several Oaxaca City or Huatulco hotels with easy access to these facilities. Use of hotel tennis courts is sometimes included in your hotel tariff. If not, fees will run about $10 per hour. Golf greens fees, which begin at about $50 for 18 holes, are usually extra.
Links:
[1] http://www.moon.com/destinations/oaxaca/oaxaca-city
[2] http://www.moon.com/destinations/oaxaca/pacific-resorts/bahias-de-huatulco