Mr. Viruzzz, the origin of the phrase "sick as a dog" probably comes from at least the 17th century and means very sick, especially with vomiting. Recall that British use ill to mean having a bad cold, but "sick" usually refers to vomiting. And if you think about how dogs are when they are sick, especially from some dietary cause, you can understand how the term came into being. In this sense, "sick as a dog" , is probably better said as "sick like
Actually Mr. CD, Mr. Federer has said that his illness is unrelated to the "virus" that has struck other players. He believes he has some form of longer lasting flu with some fever and other flu symptoms, and his whole family has been struggling with it. During the Belluci match, the cameras flashed over to Mrs. Federer yesterday semi-isolated in the stands, coughing while on her mobile phone.
Mr. Federer should probably be in bed resting, but these tennis players can be a stubborn lot sometimes, no matter how much money they've earned.