greatness is normally defined by titles won , serena williams is well down the list ,compared to king navratilova and others.
next, the standard of womens tennis is not as high as mens tennis williams is therefore not as good / great as a tennis player as federer nadal djokovic.   
williams is a jehovahs witness ,a religion who have principles about not giving blood , does anyone on this forum criticise her for her principles? yet the same tolerance is not shown to djokovic