This may have been said before, but in addition to genetic and hormonal differences between men and women (and women and women) that I mentioned earlier, there are a handful of reasons why top tennis players may not always appear to be chiseled with defined muscles. Many of the best natural (and most athletic looking) athletes compete in other sports. Those who do choose to try to excel in tennis have a better chance of progressing to the next level if they (or their parents) can afford expensive tennis lessons, which can rule out many talented athletes who have potential.
And in tennis, only a small percentage of the talent and ability that determines success involves maximum mobility to get to the ball, and maximum endurance. Just as important, if not more important, are being able to hit hard and accurately and to have exceptional hand-eye coordination, to have excellent shot placement and a mastery of various shots, and to have the knowledge and wisdom to anticipate what your opponent is going to do and to determine your opponent's strengths and weaknesses. Athletes who are superior in those aspects can still be more successful in tennis than athletes who can move quicker or who have better endurance due to lower body fat... The type of surface that a tennis match is played on (hard, grass, clay) makes a difference in the chances for success of a particular athlete too.
Because of those things, it's possible that an athlete who may not be in top physical condition or be the most mobile athlete can keep winning, at least until she routinely has to play against the Serena Williams types who may have even more of the traits that make up an outstanding tennis athlete. But what a tennis player can do with the ball once it reaches her racquet, and her ability to anticipate what her opponent can and will do, could allow an athlete who appears to be in slightly inferior physical condition to still dominate in the sport.