There's a reason Serena Williams is the No. 1 player in the world. It's not because of her serve, which is superior. It's not because of her speed, which is stunning. Serena Williams has mastered the mental game of tennis.

Because Serena's game is so powerful, fans often don't see her moving opponents around like chess pieces similar to former No. 1 Martina Hingis. It just doesn't take Serena that long to win a point.

But it's not her power that wins her matches. It's her mind. Somewhere, likely right after losing the 2001 U.S. Open final to her sister, Serena Williams decided she was "tired of losing."

Well, aren't we all? But that's the difference between everyone else and champions. Serena decided she was tired of losing, so she stopped. That simple.

Wow. Where can you get some of that? It's what everyone who watches sports wants to know. Where can you find that edge? How can you maintain that belief in yourself? Serena's found the way. She's so confident she's accused of being cocky. She's so confident that her goal for this year is to be perfect -- to not lose a single match.

And so far, that belief is keeping her on top of the world. It's not just her ability to raise her game to another level -- making only five unforced errors when she was down 5-1 to Kim Clijsters in the semifinals. Serena believed that she could dig herself out of that hole. That she could win six games in a row.

And beyond the belief is her desire. After earning the U.S. Open title in 1999, Serena got close to another major title twice and each time her sister Venus got in her way. But once Serena decided that she wanted to start winning, she's beaten her sister Venus in every meeting since.

Just last year, their head-to-head record went from Venus leading it 5-2 to Serena grabbing the edge 6-5. With this Australian Open victory, Serena extended her lead to 7-5. Venus is incredibly talented. Some might say it's unfortunate that it appears to be slightly behind that of her sister. But who knows if either sister would have been as good without the other. They always have a Grand Slam tournament champion to practice with. They push each other on. Just the other day, Venus set the record for the fastest serve at the Australian Open. Serena said later that she tried to top her in her next match.

Today, Serena holds the edge. Venus has lost five consecutive matches to her sister. Besides a stronger serve, the reason can be seen in Venus' face. As she watched Serena in her comeback against Clijsters, Venus applauded with a look on her face that was just like any other fan in the stadium -- complete amazement at Serena's ability to win an incredible point. If Venus is going to beat her sister again she's got to rediscover her belief.

In the meantime, others are hoping to topple both Williams sister. It is only going to get harder for Serena as players like Clijsters and Justine Henin believe that on any given day they can still beat her. But Serena, at age 21, still has room to improve even more. And she seems to know it. Before her semifinal with Clijsters she said: "I just have to be able to make sure I am in my 'A' game," before adding, "I've never played an 'A' game in my career."

That ought to scare the pack chasing her.

For now, Serena's the one who is winning -- and making history. Only five women have ever been able to say that they were the defending champion of each Grand Slam tournament. And although some in the tennis world are still pointing out that she hasn't earned The Grand Slam, as soon as Serena hoisted that trophy in New York last September the media started salivating over what we dubbed the "Serena Slam."

Why? Because this doesn't happen often. Five times in the history of tennis -- not just the Open Era -- the history of tennis.

And the next story could be a Grand one. All Serena Williams has to do is stay healthy and keep believing. One down. Three to go.

Cynthia Faulkner is the tennis editor for ESPN.com.