In Part 4 of Joel Drucker's key moments of the Open Era, he ranks the best stories of the turn of the century. The sisters sit, just as they must, at the top:
The success of Venus and Serena Williams is one of the greatest stories in the history of sports. At least the Manning brothers grew up the children of a quarterback and in an affluent part of New Orleans. Venus and Serena were raised in Compton, Calif., quite a distance from lily-white tennis clubs. Their father Richard had proclaimed they would in time rule the sport. And for just under two years, they did. From the 2001 U.S. Open to the 2003 Wimbledon, the two met in six of seven Grand Slam finals. Even beyond that, though, Venus and Serena have won a combined 16 Grand Slam singles titles. Just imagine Tiger Woods' brother chasing him down at Augusta and you'll see just how amazing this story is.
Of course, Roger Federer and Rafael Nadal are also on the list at No's 2 and 4, respectively, but I found his discussion of equipment the most interesting.