(Undated)  --  One of the most successful tennis players of all time won't compete in the upcoming Olympic Games.  Roger Federer announced on Twitter he's withdrawing from the Games that begin later this month.  He blamed a "setback" with his knee following two surgeries on his right knee.  The Swiss superstar said the issue happened during the recent grass court season.  He played at the just-completed Wimbledon tournament, but lost in the men's quarters.