Tennis does about 10 in-competition tests a year, on average. These are completely useless, since the players know they're coming.
It also does an average of ONE out of competition test a year. This is hilarious - top cyclists get 30+. Players can also skip two in an 18 month period without any penalty.
All these details are published on the ITF site.
For example, for last year we know Nadal had "1-3" out of competition tests (they do it in bands for privacy reasons). Almost all of those "1-3" mean one, since we know the total number of tests.
Even sillier, almost all of the tests are urine. Blood tests are needed to detect a lot of substances.
There are numerous other mammoth loopholes, like slams only testing after a match. HGH/EPO in the right doses are only detectable for 10-12 hours, so players can dope themselves silly on off days.
The whole system is a complete joke.