I've read some of the news stories, the Freakonomics chapter on sumo wrestling and one post on the topic on the Freakonomics blog, so I don't have much of a leg to stand on when I say this, but what the hell: Match fixing in tennis is not a problem but doping probably is.
Match fixing has been in the news a lot but I can hardly think of a case of a famous tennis player being caught doping, so I say that in the face of the evidence. But tennis is just a terrible sport to fix. The incentives line up so well in tennis. Money, ranking and status in tennis go with winning every match you can (this isn't true in sumo and in lots of other sports). But more importantly, people just don't bet much on tennis, especially on matches involving lower ranked players, so there's not much money to be made. Matches that are heavily bet on stick out like a sore thumb.
I do think it's likely that Davedenko threw his match way back, but I'll bet it was more from people credibly threatening to kill him than him making lots of money. I have no reason to think this beyond the fact that he's Russian. Sorry.
On the other hand, for some reason people taper off drastically at quite an early age, much more so than most other sports. So physical intensity is very important and the benefit of doping is potentially very large. So players probably do it.
Good thing Federer is so scrawny, otherwise I'd wonder.