It's a debate Serena Williams instigated after being overtaken as No. 1 in the world this spring and further fanned after winning Wimbledon in July.
Who is the best player in women's tennis? Is it the athlete who wins most often? Or is it the athlete who wins when it matters most?
The sport's computer-based ranking system rewards the former - for all but 11 weeks this year, Russia's Dinara Safina, who has toiled like none of her peers to achieve her life's dream, recognition as world No. 1.
But Serena has made the more forceful case, crushing nearly every competitor en route to claiming two of the season's first three majors (the Australian Open and Wimbledon).
The younger Williams sister is unapologetic about her pursuit of the titles that make a sporting legend, flouting expectations that she compete at her best - or even compete at all - at mid-level tournaments that lack the majors' cache.
No player in tennis thrives quite like Serena under the sport's brightest spotlight or on its biggest stage. And that's where she returns this week - the U.S. Open and the glitz of New York - as the tournament's three-time and defending champion, albeit its No. 2 seed, slotted behind Safina.
And if Serena wins the 2009 U.S. Open to claim three of the season's four majors (and four of the last five), it would bolster her assertion, issued in May, that, "Quite frankly, I'm the best in the world," and make a mockery of a ranking system whose credibility is already under fire.
On the men's side of this year's U.S. Open, the presumptive favorite is also the defending champion - Roger Federer, who elevated the notion of what's possible in men's tennis yet again this summer by claiming his 15th major title with a five-set victory over Andy Roddick at Wimbledon.
In doing so, Federer reclaimed the No. 1 ranking he had ceded to Rafael Nadal last August and bolstered his case as the greatest player of all time - status four-time U.S. Open champ John McEnroe accorded him even before his latest feat.
Between them, Federer, 28, and Serena, 27, boast a staggering 26 major titles and eight U.S. Open crowns. But that's not to say that this year's tournament lacks intrigue.
Roddick arrives as the sentimental favorite, having won new respect for the grit he displayed in the Wimbledon final, which ended on the longest fifth set in Grand Slam history, with the Swiss prevailing, 16-14.
It was a magnificent battle. And Roddick's grace amid the heart-rending defeat revealed a new dimension in a once-cocky player.
"It's like what happened with Andre Agassi," said former pro Mary Carillo, who'll join McEnroe and Jim Courier in the CBS booth. "It's not that age is always ennobling, but they both became such grown-ups in later years."
Roddick, 27, the last man other than Federer to win the U.S. Open (2003), has improved his fitness and varied his game in pursuit of the second major title he covets.
"He's a guy who's fighting to get himself back in the conversation," Courier said. "It's not getting easier for him, but it seems like he has a new wind at his back."
He'll need it. Men's tennis is suddenly flush with talent - young, hungry players who excel on multiple surfaces. Among them: 22-year-old Andy Murray of Scotland, last year's U.S. Open runner-up; 22-year-old Novak Djokovic of Serbia; and 20-year-old Juan Martin del Potro of Argentina. Any one of them could advance to the final.
There is also Nadal, 23, who recently returned to competition after a two-month layoff forced by tendinitis in both knees.
Nadal's absence arguably made Federer's resurgence possible. With Nadal falling in the fourth round of the French Open, Federer went on to win the one major he lacked. In each of the four years prior, it was Nadal who handed Federer his only losses at the French. And the Spaniard still holds a 5-2 edge against him in the finals of majors.
But it's unclear if Nadal's knees can withstand the pounding of seven best-of-five-set matches on a hard court, the least conducive surface to his game.
On the women's side, the only certainty is that there won't be an all-Williams final, with Venus and Serena in the same half of the draw.
That's a pity for CBS, which moved its broadcast of the women's final to prime time in 2001 largely because of the staggering audience the sisters command and the expectation that they'd settle the title between them for years to come.
This year, they're on track for a semifinal clash.
Safina, meantime, is desperate to prove she can win her first major, having thrice finished runner-up. Nerves proved her undoing in each loss.
Fellow Russian Elena Dementieva, 27, also seeks her first major and has played the best tennis of her career in recent months, nearly toppling Serena in the semifinals at Wimbledon. Still, questions abound about her serve.
But the biggest surprise could come from Belgium's Kim Clijsters, 26, who only recently returned to tennis after a retirement that lasted two and a half years.
Even Serena, from whom compliments don't come easily, confessed she has been "a little shocked" at Clijsters's swift return to form.
"She hasn't lost a step," Serena said of Clijsters, the 2005 U.S. Open champion. "She hasn't lost anything."