How long can Serena remain dominant?

Am I the only person who thinks Serena Williams is skating on thin ice? She's pulled out of the Pan Pacific Open, despite reports that she returned to the practice court (following surgery on a foot she injured shortly after she won Wimbledon) earlier this month. Leaving the issue of her health and fitness out of it, this is disappointing if not entirely unexpected news.

Serena has made a habit of gliding across the WTA pond, darting among the trees and islands for long periods before bursting forth to throw in an amazing leap or mind-blowing flip that enables her to retain her place as the premier performer in the game.

All credit to her for that. Keeping the highest possible ranking while doing the least amount of work is an amazing feat that requires enormous talent, if not the most highly developed work ethic. But before you rush to judgment about Serena's dedication, keep in mind that she's played one more tournament than her only true rival, Kim Clijsters. It's not like either of them is stealing money. Or maybe they both are, which is not a particularly difficult assignment in the WTA these days.

But you have to wonder: How long can Serena survive as the dominant player if she doesn't enter -- and win -- more tournaments? She was somewhat lucky this year when Clijsters missed the French Open, and played like a chump in Australia and at Wimbledon.

The two Grand Slam events Serena won last year (the Australian Open and Wimbledon) have kept her at No. 1, but barring an exceptionally strong finish in the last three months of the year, Serena will be under enormous pressure to defend her Australian Open title, for no better reason than the ranking points at stake.

With 6,995 points, Serena enjoys a comfortable lead over Caroline Wozniacki, who has amassed 5,910 points without winning a major and while playing in 24 events, a whopping 10 more than Serena. That 1,000 point differential could easily be wiped out if Serena has a disappointing Australian Open and Wozniacki a good one (a Grand Slam win awards 2,000 ranking points, and each year the points earned the previous year fall off the rankings table).

Serena is ranked No. 1 because of the (correct, in my opinion) emphasis placed on performance in Grand Slam events when it comes to ranking points. But it's telling that she's No. 2 in the "race," the redundant and relatively obscure contest to qualify for the year-end championships on the basis of points earned at tournaments. She now trails the same Wozniacki in this race by about 250 points. And Clijsters and Vera Zvonareva are hot on Serena's heels -- closer to Serena than she is to Wozniacki.

Nobody in his or her right mind cares about this bogus "race," but it does underscore the increasing emphasis the tours place on consistent performance. The tours want you to play a lot of tennis, in order to advance the product. It takes a player of exceptional ability, like Serena, to protect herself from this pressure by winning the tournaments that count the most. But it also means that, barring an unlikely late-season surge this year, anything but a successful defense in Australia and at Wimbledon in 2011 could have an enormous impact on Serena's ranking.

To borrow a term from the banking industry, Serena is highly leveraged, although in her case the capital has been earned, not borrowed. But it can still vanish, quickly.

Serena could take a hedge against an unexpected downturn by banking points this fall, but I wouldn't bet that she will. She plays a high-risk game in more ways than one.