I'm surprised nobody has mentioned Serena in this conversation. Looking at photos of her over the years the increase in muscle mass is astounding. And there's the now infamous "safe room incident" and speculation about her foot injury being a cover for a silent ban.
Serena has more muscle and is cut more than Federer who is fit but otherwise has a plain old fashioned tennis player's physique. In a Celebrity Death Match I think Serena would easily beat Federer!