In several, very fundamental ways, they are the same extraordinary person.
Thirty-two years ago, Serena Williams and Roger Federer were born within seven weeks of each other. They are, at the very least, the tennis players of their generation, ruthless competitors when matches mean the most. Appropriately, they both own 17 Grand Slam singles titles.
"It's an honor to be even with Roger," Williams said after winning last year's US Open. "It feels really good to be in the same league as him. He's just been so incredibly consistent, so we have had really different careers."
Indeed, their journeys to the summit couldn't be more different. Williams learned to play on the public courts of Compton, Calif., while Federer, who was born in Basel, Switzerland, followed the more conventional path of an elite junior. Williams' slashing style is predicated on unprecedented power. Federer's game is an astonishing combination of variety and finesse.
Here's another difference: While Federer won all of his major trophies in just over nine years, Williams' swath spans 14 years and will almost certainly continue. This will be the Swiss champion's 57th consecutive Grand Slam; since winning her first, Williams has missed 10 majors -- at least one in each of eight seasons.
And there's this: Federer, ranked No. 6 among ATP World Tour players, heads into next week's Australian Open with a new coach (childhood hero Stefan Edberg) and a larger racket head (a 98-square-inch Wilson) than he's ever employed at a Grand Slam event. Last year was the first time in 11 seasons he didn't reach at least one major final. Williams, meanwhile, is the WTA's No. 1-ranked player and is favored to win this and, quite likely, every Grand Slam of 2014.
Practicing with hitting partner Sascha Bajin under the eye of her formidable father, Richard, last month in Florida, Williams asked herself what has, in recent years, become a rhetorical question:
"Why am I, like, doing this so many years later?" she related a few weeks ago at the 2014 kickoff tournament in Brisbane, Australia.
"I can't stop," said Williams. "I love being out there. I love competing. It gives me something to do. For me, it's just about motivating myself and trying to reach new goals."
Translation: Winning more Slams.
"Oh, that's going to happen," said 18-time Grand Slam singles champion Chrissie Evert from her home in Florida. "But the thing I wonder is if she can match the tennis and the enthusiasm that she had last year. It took a lot out of her. It was almost like playing three years. Every week, the focus was on her and the emotions of being excited about her place in history.
"It's not about Azarenka and Sharapova. Can Serena do it, day in and day out? It's happening to Roger Federer. It chips away, and suddenly you're not hungry. There's a point where you don't jump out of bed anymore to play a match."
Not only is Williams the oldest player ever to be ranked No. 1, but she has also been the WTA's best player for the better part of the past two years.