Tennis Feature: Serena Williams, six times a grand slam champion and the highest-paid woman athlete in the world thanks to her $40 million Nike contract, no longer describes herself as a tennis player.

"I am," says the sometime-actress smiling sweetly, "an entertainer." Venus, the more reserved half of a duo known in their early days as the Sisters Sledgehammer, is meanwhile rumoured to be thinking of changing her name to one of African origin and setting off on a journey of self-discovery.

Makers of history and makers of myth: it has always been extraordinarily difficult to know quite what to believe and what not to believe about the two Americans. And, as neither has hit a competitive shot since Serena defeated Venus in last year's Wimbledon final on July 5th, the line between their tennis careers and their celebrity status has become ever more blurred.

There have been extenuating circumstances: Serena underwent an operation on her left knee in August and Venus needed an extended rest to recuperate from a long-standing abdominal problem. On top of this their half-sister Yetunde was murdered in Los Angeles in September.

This week Serena withdrew from the Australian Open. At the time of her operation the surgeon said he expected her to be out for six to eight weeks. Now nobody is certain when she will return. Venus, meanwhile, warmed up for Melbourne with an exhibition event in Hong Kong.

On the apparent "advice" of the WTA she has been seeded number three for the year's first grand slam event, even though her actual ranking is 11. Small wonder many believe it is one rule for the Williamses and another for everybody else.

The development of the sisters, from hugely promising teenagers into an all-conquering pair, has always been complex, with their father, Richard, regularly muddying the waters. But these past six months have amounted to something different.

Russia's Anna Kournikova had previously proved that you do not have to win titles to become a millionaire megastar; Serena and Venus have now demonstrated that you barely have to play at all, which is all rather unfortunate for tennis.

But this is the way the women's game has progressed, should it be deemed progress, during the past seven or eight years, with on-court rivalry playing second fiddle to celebrity status. Perhaps this is not surprising given the huge amount of money involved at the top of the women's game, and the nature of those who have been dominant.

Switzerland's Martina Hingis, who retired at 22 with five grand slam titles, loved the celebrity life, and the Williams have followed the same path, albeit in a rather more spectacular and flamboyant fashion. Sport has become but a stepping stone to stardom.

Justine Henin-Hardenne and Kim Clijsters, number one and two in the world, are temperamentally different from each other, yet have more in common with the former multi-champion Steffi Graf than either of the Williams. The Belgians' rise to the top is patently via professional ambition, whereas Serena and Venus have always stressed that tennis is not the be-all and end-all.

As Serena said, after making her acting debut: "Balancing the two [acting and tennis] is something I'll have to do. It's a totally different world and far more glamorous than tennis but I love tennis because it's a big part of my life."

Just how big is the question that nobody outside the immediate Williams family can answer, for either Serena or Venus. At various times there has been speculation about their playing futures - first over Serena when her elder sister was dominant, and now Venus. It is one long tease.

During last year's US Open the two sisters, both unfit, turned up on seemingly endless breakfast and late-night TV chat shows, upstaging what was going on at Flushing Meadows while at the same time pledging to remain at the top of the circuit for another decade.

"I'm not going to retire before I'm 33 and Serena and I will have to retire together because otherwise I won't have a doubles partner," said Venus with her usual disarmingly wistful smile.

Few believed her. Both have always restricted the amount of tennis they have played annually, confident in the knowledge that they could overpower and outplay any rival when it most mattered, regardless of the length of their lay-off. As their fellow American Lindsay Davenport said in Australia: "They've proven everyone wrong in the past by coming back and doing great after a long lay-off. They are great athletes."

Serena Williams may have lost the number one spot to Henin-Hardenne but she stands head and shoulders above everyone as the most marketable female athlete.

"And her foray into acting should only heighten her appeal," said Abraham Madkour, executive editor of the Sports Business Daily and Sports Business Journal.

"Venus has all the same qualities as her sister but it's a little bit like, 'we couldn't afford Serena, so we took Venus' if you're an advertiser," said Matt Lalin of Steiner Sports.

Yet if one Williams sister represents a proven revenue winner for advertisers, together they remain doubly potent.

Only Michael Jordan, Tiger Woods and Lance Armstrong are ahead of the combined endorsement power of the sisters in the US market.

Generally Serena, because of her foray into acting, has been the more visible since Wimbledon. She made her debut playing a kindergarten teacher on the ABC prime-time sitcom, My Wife And Kids, and then came her first appearance on cable TV in Street Time. She played a reformed member of a gang wrongfully convicted of cocaine trafficking, shedding more tears, it was remarked, than she ever seemed to over her knee operation.

She also took the awards for US athlete of the year and women's tennis player of the year only 12 months after accepting both on behalf of Venus.

And together they have appeared in toothpaste ads, chewing gum ads, the launch of Serena's designer clothes, Aneres (Serena backwards), and then, in the footsteps of Paul Newman and Dustin Hoffman, for a McDonald's gig alongside Hamburglar.

There seemed no need of tennis, and tennis could only watch and wait. "But I'm going to be a much better and stronger player when I get back. I'll be back with a mission," Serena gushed before Christmas. Then her withdrawal from the Hopman Cup, one of the pre-Australian Open warm-ups where she was due to parade her new Nike gear, spawned fresh doubts.

She has been seen regularly at LA Lakers games, recently attended a function in support of Michael Jackson and last Tuesday spent the day doing a Nike photo shoot at the Mountain Gate Tennis Club in Los Angeles. Word there was that she would play the Australian Open. Word elsewhere was that only Venus would be in Melbourne and that proved to be correct.

The cynical point out that Venus needs to renegotiate her Reebok contract, that she has not won a grand slam event since 2001 and that she has slipped out of the top 10. Success in Australia, in the absence of her sister, would obviously boost her bargaining power and keep the endorsements ticking over nicely.

The truth is that nobody really knows. The US market is vital to women's tennis and, as such, the Williams sisters know they can do what they like in terms of when and where they play.

It is an uncomfortable situation for tennis and one that will continue as long as Serena and Venus wish to compete and can remain impregnable at grand slam level.