Serena's standards may never be matched

Before Serena Williams moves on from completing a second 'Serena Slam' to pursuing tennis' first true calendar grand slam in more than 25 years, it is worth pausing to appreciate what she has done.

First of all, there are the statistics. And what statistics they are:

She's won 21 Grand Slam titles; only Steffi Graf, with 22, has more in the Open era of professional tennis (the all-time record is Margaret Court's 24).