So what DOES makes a major tennis champion?

Through the inevitable upsets, close calls and predictable results that characterize every U.S. Open, even the players don't exactly have the answer to what makes a major singles champion.

But it is an interesting question, and all of them - in one way or another - have to deal with it.

Merely the ability to defeat a top player on the sport's biggest stage hardly solves the riddle. Serbia's Janko Tipsarevic, with the relatively humble world ranking of 44th, displayed a marvelous mix of tennis craftsmanship and decision-making in knocking former champ Andy Roddick out of the tournament Wednesday night.

Tipsarevic then proceeded to lose his next match to No. 17 Gael Monfils. "I am playing really well on big stadiums against big players," Tipsarevic said. "But the difference to become a champion in this sport is that you need to play this level for not just one match, just one tournament, but let's say for a period of a year.

"Or, in the case of Nadal and Federer, just all year long. I'm telling you, it's really, really hard, and only champions can do it."

So the key is consistency at the highest level? And the source of that is?

"You have to be switched on from the beginning of every tournament," Andy Murray said, "because all of the players play very, very well." Murray has figured out how to get to No. 4 in the rankings and be considered, for several years, a threat to win a major title.

But he still hasn't pulled that off.

"Mentally," said Kim Clijsters, last year's U.S. Open champion, "you just always have to hang in there, whether you're playing good or bad . . . focus on the little details and just always try to know what's ahead and what you're trying to aim for."

Maria Sharapova, three times winner of a Grand Slam event (including the 2006 Open) and making another run at this U.S. crown after a year of battling injury, often has been cited for her toughness under pressure. "Growing up," she said, "I was never the biggest, the strongest girl on tour. Even though I have big shots, there are many girls that are more powerful, that are quicker. I have to battle it out, find a way to stay calm and be tough inside of me."

That leaves top-seeded Caroline Wozniacki, last year's Open runner-up and seeking her first major title, puzzling out how to out-tough (or outwit or otherwise outplay) Sharapova in their quarterfinal match Monday. "You know," Wozniacki said, "she's won Grand Slams. I haven't."

Federer offered that "it's hard to pinpoint what makes you a better player. I just think maybe your work ethic, maybe you're more professional, try out different things. The mind-set."

Rising 22-year-old American Sam Querrey, through to a major singles quarterfinal for the third time, reasoned the solution is all tangled up in experience at the top. "A lot of those guys," Querrey said, "Federer, Nadal, you know, for the last five years they've been in the semis and finals of Slams, every Slam, basically, so they've been there before.

"Let's say I were to jump into a semifinal of a Slam: It's new for me. I might be a little nervous. A lot of it is mental. Those guys are strong in the head and they don't ever give away free points.

"A lot of it is skill, too. I mean, Federer hits those 'tweener shots at will. I can't do that."

So. The answer is between the ears, and being able to hit between the legs?