MELBOURNE, Australia — Serena Williams has won eight major titles and has been ranked no. 1 in the world, and there’s no doubt she’ll be remembered as one of the game’s great champions, and perhaps its most talented player in history, when she retires. But she has never faced a challenge on the tennis court like the one she faces now.

The prize in women’s tennis today is not the Australian Open title, which Williams won in surprising fashion last year and is in position to defend this year, now that she has reached the quarterfinals without losing a set. No, what’s at stake this year, beginning with this tournament, is much bigger than any title: It’s greatness, or more precisely, the degree of greatness Williams will be accorded years from now if she doesn’t stare down Justine Henin this year and dominate the tour.

In her best days, from 2002 to 2003, Williams wasn’t challenged by anyone, not even her sister Venus. She served too well (more on that in a bit), hit the ball too hard, and moved too quickly for anyone to so much as annoy her. When she fell from the top, it was easy to excuse her exit and put an asterisk beside the names of the women who began to outplay her. Williams didn’t practice as much. She gained weight. She became depressed over the shooting death of her half sister. She played well at times, but not for enough days in a row to win consistently.

Last year’s Australian Open changed all that. Williams won that title despite terrible conditioning. By her mother’s own admission she arrived in Australia out of shape, yet her strokes, especially her serve, and her competitive zeal are such that she still ran roughshod over some fine players, including a drubbing of Maria Sharapova in the final. In terms of talent, she showed us again, there is no one like her. Since then, she has played more and worked herself into better shape. Perhaps her commitment isn’t as strong as some would like, but there’s no doubt that she has become increasingly fit since Melbourne last year and is eager to win titles.

The only problem is, she isn’t winning them.

For the first time in her career, Williams has found that a good effort is not enough to make her the best player in the world. In her early years on the tour, her sister Venus was better. In the years after her dominant period, she wasn’t committed to the cause. There can be no excuses now. Henin has upped the ante in women’s tennis, and Williams has so far failed to respond. She played 10 tournaments after last year’s Australian Open and only won one and that one, in Miami, was a shocker (she trailed Henin 6–0, 5–4 and was two points from losing the match before she rallied). It has become clear since then that Henin choked in that match as much as Williams played well. The two met again at the French Open and Henin won in straight sets. At Wimbledon, Williams’s best tournament, she took a set from Henin but lost. At the U.S. Open, Williams’s home tournament, Henin won a close set and then pummeled Williams 6–1 in the second, leaving no doubt which woman was the best player in the world (Henin also defeated Venus and eventually won the title). Williams was in a foul mood after the U.S. Open loss and said Henin had hit “a lot of lucky shots”; nothing could have been further from the truth.

Not long after she arrived in Melbourne, Williams agreed that Henin had “pushed the bar really high.” Does she have an answer? We might find out soon, if Williams and Henin meet in the semifinals of the Australian Open later this week. Neither player has dropped a set so far; neither player has played her best, either. Henin has the more difficult road, as she will have to defeat Maria Sharapova in the quarterfinals while Williams takes on Jelena Jankovic, who has played poorly so far and continues to complain of soreness and nagging injuries. If Henin wins a fourth consecutive time, Williams will know that she has to do something more to regain her place as the premiere women’s tennis player in the world. As it is, she is relying too much on two weapons, her serve and her return. She and Henin are near equals when it comes to returns, but as far as serves go, no woman has ever hit one better than Williams (Venus owns the speed records, but her serve isn’t nearly as reliable or accurate as her sister’s).

How good is Williams’s serve? Just ask Mardy Fish, who played mixed doubles with Williams at the Hopman Cup earlier this month.

“She’s got as good a serve as anybody does, better than a lot of the guys,” Fish said. “She serves up to 120 mph almost every time. It’s amazing how much better she is — I don’t know if I should say this or not but I will — it’s amazing how much better she is on the doubles court than the other girls. She can just dominate. She’s not a weak link in mixed doubles at all.”

Jankovic, ever humorous, said Williams’s serve belonged on the men’s tour. “She’s a strong girl,” Jankovic said. “She’s stronger than me twice — twice my size. Sometimes I feel like she’s going to blow me off the court.”

In four matches so far, Williams has served 36 aces and won 85% of points on her first serve. She hasn’t fared as well on her second serve, though (she has won 49% of those points and been broken four times). If Williams only serves as well against Henin as she did against the far less consistent and mentally tough Nicole Vaidisova, she probably won’t win because the rest of her game, though improved from last year, isn’t where it needs to be. There’s still a lot of time for her to get it back — Williams is, after all, only 26 years old, plenty young enough to reclaim her place at the top of the game and stay there for a little while longer. It is just going to be a lot more difficult to do than she might have expected.

Mr. Perrotta is a senior editor at Tennis Magazine. He can be reached at tperrotta@tennismagazine.com.