Serena has the perfect balance of excellent technique on her serve and groundstrokes, lethal power, and uber-athleticism.
Graf is often praised for her steely demeanor on the court, and she should be. You could never tell if Steffi was up 6-0, 5-0 or down match point.
But what separates Serena is her will to win. Watch her matches when she is down in a second set after losing the first, only to roar back. She plays with a sense that it is personal to her, especially against her top rivals: 17-2 against Sharapova (last loss was 2004), 14-3 against Azarenka, etc.
In fact, for her entire pro career (from 1994-2015), Serena has a record of 139-56 when playing an opponent ranked #1 in the world. Think about that. For 21 years, no matter where she is ranked, Serena will beat the #1 ranked woman in the world in 71.3 percent of matches.
And her career resurgence at her age is nothing short of amazing. On February 18, 2013, at the age of 31, Serena Williams took over the top spot in the WTA Tour rankings. She has not budged since. Since that date, she has won 19 tournaments, including four Grand Slams, while compiling a record of 138-10.
Graf was my favorite female tennis player growing up (Ivan Lendl was my favorite male player). I even modeled my forehand off of Steffi, with the big swing and running around to hit the inside-out shot. But Serena has become not just my favorite female tennis player...not just my favorite tennis player...but one of my all-time favorite athletes, regardless of sport.