Tennis does about 10 in-competition tests a year, on average. These are completely useless, since the players know they're coming, so make sure they have nothing in their systems.
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. This means when they are doping, they simply lie about where they are so as not to get tested. Sports that actually try to catch dopers do enough tests that they rapidly get to that two point and can't skip tests any more. Tennis doesn't, so it doesn't.
All these details are published on the ITF site.
For example, last year Nadal had "1-3" out of competition tests, and the same in 2010 (they do it in bands for privacy reasons). We know that most of those "1-3" mean 1, as they publish the total number of tests done.
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.