The question now is not of performance, but of endurance. It's a question about legacy ever so much more than celebrity.

	Can Serena ever hope to achieve greatness when it is Venus who stands as her only rival on the other side of the net?

People don't ask whether Serena Williams can play tennis at the top level. What they do ask, or at least what they should ask, is whether Williams can make herself ultimately remembered in a sport she already commands with such evident ease.

You win everything in sight for a couple of years, and congratulations: You're Martina Hingis. So what does it take to become Martina Navratilova?

Genuine rivalry, it says here. And what's so fascinating about that notion is it is one of the very few areas on the court Serena Williams honestly cannot control.

It's amazing longevity should come into the picture so quickly for Serena. When a woman has achieved what Williams has achieved at the reasonably young age (in tennis) of 21, then the logical extension of the thought is how long can she continue to so achieve.

It is the ultimate compliment, of course, because it assumes elite status already. It helps when you hold all four Grand Slam singles titles concurrently, as Williams does now after her Australian Open championship recently. But it's also the land mine along the road to a legacy that might endure. Some of the finest players in the modern women's game also have had some of the most truncated competitive careers, which is exactly what makes an athlete like Navratilova, who just won her 57th title at a Grand Slam tournament, shine so bright.

It's hard to know yet where Serena Williams is going to fit. She exists in a time and place for women in sports and sports celebrity that is unlike any before it, which is a fancy way of saying she can be famous for years just on what she already has accomplished.

She is one of the top-achieving African-American women in the sports universe. She has successfully wrested the title of Dominant Active Player from her big sister, Venus Williams, who ought to serve as an object lesson in the shelf-life of such things, as Venus's self-described ambivalence toward her tennis career clearly cost her that spot at the top of the ladder.

And Serena Williams is now going to deal with an elite athlete's level of distraction, both commercial and personal. She is going to deal with the weight of her own achievements, since all of her future seasons will be measured against the glorious work she has done over the past year. She is going to deal with the persistent demands of corporate sponsors like Nike, which reportedly is willing to pay her $50 million over the next several years to shill its products.

	Off the court distractions present another hurdle for both Venus and Serena.

She will, on the most basic levels, have to confront the question of just how much she wants tennis, of how hungry she is. Her sister lost her appetite last year, spoke of retiring, and allowed Serena to pass her up. Will Serena similarly become bored with the grind, unwilling to put in the phenomenal amount of work it takes to stay on top?

Absent a real and consistent rival -- and, no, Venus doesn't count -- that's a solid possibility. What the last four majors have demonstrated (Melbourne's three-set final duly noted) is Williams vs. Williams makes for a decent storyline, a clear conflict of emotions, and seldom great tennis.

No, we're thinking of something more in the Navratilova-Evert mode, or perhaps the later Navratilova and the young Steffi Graf, or even Graf-Seles. We're thinking of a player so good and so hungry that even a small slip by Serena Williams would open the door to a challenger. Bjorn Borg had Jimmy Connors; they both had John McEnroe. Larry Bird had Magic Johnson; they both had Michael Jordan.

How Pete Sampras is remembered in tennis may ultimately have something to do with the perceived lack of elite opponents during his best years at the top -- not strictly true, of course, but true often enough. Andre Agassi is enjoying great success now late in his career, and you almost find yourself wishing he had been in this form during Sampras' dominant period, if only because Agassi could have been his foil, year-in and year-out. Sometimes the legacy of the champion is forged from the class of his rivals.

A true peer could keep Serena Williams interested -- and sustained interest is no longer a given in a sports world that worships celebrity buzz to the same extent as any entertainment-driven business you'd care to name. Neither are health and incident-free seasons a given; the former has brought down Hingis, while the latter changed Monica Seles' career in brutal and unyielding ways.

Assuming Williams stays healthy and happy, though, this really is a question of longevity. It is a question, simply put, of how long she can be this good. She's a fine player, an energetic force, a potentially dominant singles champion. But she could do with some help.

That part about her future is almost completely out of her hands. And it may be the part that significantly defines where she goes from here.

Mark Kreidler is a columnist for the Sacramento Bee and a regular contributor to ESPN.com