McEnroe is not right! There is no "right" answer when comparing apples with oranges. Following McEnroe's logic, a lightweight boxer could not be considered the world's greatest boxer, due simply to the fact that his physical attributes are not the same as those of a boxer in the heavyweight class.  Obviously, that's completely false. To make a fair comparison, you need to compare apples with apples: records, winning percentages, achievements, tennis strengths and weaknesses (i.e. facts, hence Williams' comment). When you start making comparisons on an uneven basis, you're going to end up with a meaningless answer based on sheer speculation.