No one is accusing Federer specifically. But it seems very likely that doping is common in tennis and so no one is excluded.
You seem to suggest that tennis players don't have enough to gain by taking PED, whilst the risk is too great. The risk of getting caught it pretty much nil. And any tennis player could benefit, after a long 5 setter, from some blood transfusions, in order to recover quickly and be ready for the next match. They're not doing any blood tests, so there's really no risk in trying blood doping but plenty to gain.
Do you really think there are no other ways to draw attention to yourself other than routine testing?