Muscle tissue does not equal strength, I know body builders who cannot lift as much as regular gym members who do strength training. Tennis player with the most impressive physice is Canas, he has excellent lats, chest, ab and oblique muscles.
You cannot tell who is "the strongest" from just a view of their body, somebody may be built like Ronnie Coleman (in my avatar) and not be able to lift as much as a man who appears much smaller.
In this case, either Safin or Roddick, they clearly have a larger natural frame than both Dancevic and Djokovic.
Djokovic- a mistake
Roddick- He always claims he has the "breadstick arms," but clearly has some power cuz of that booming serve
Safin- I would say is strongest
Dancevic- Why do many people not think so???
As for others, of course Monfils, Cañas, etc...
Perhaps I will include them in a later poll, I just wanted to see out of these 4 players