Keith Duggan Sideline Cut: Black is what strikes you most whenever the Williams sisters appear on a tennis court.

At Wimbledon, particularly, their ebony skin makes a statement that always leaves even their most fashion expressions in the shade. Maybe it is because of the all-white dress code that still prevails at Wimbledon or the flaxen English skies and the overwhelming whiteness of the crowd but every time they have stepped out onto the green courts, you are instantly reminded the Williams sisters are different.

You don't have to listen too closely to get the impression the professional tennis set does not quite know what to do with the Williams cavalcade. There are obvious parallels to be drawn between the Williams girls and Tiger Woods, athletes redefining the limits of sports that have traditionally been the preserve of WASP culture.

But even though old Earl Woods remains a bit of a loose cannon he could always be forgiven because of his military life. In America, all decorated men of war are encouraged to turn a little ornery in their old age. Anyhow, Tiger learned how to become a stellar member of golf's ruling academy.

Rather than challenge or upset golf's ruling tradition he has become its most immaculate member. For that reason - and because of his omnipotent presence through televised golf tournaments and advertising campaigns - Tiger's colour, his blackness, is hardly noticeable.

I don't think this will ever be true of the Williams sisters. The best part of watching the Williams girls mowing through the game's elite for yet another all-family Grand Slam confrontation has been watching the supporting cast in the stands. They are evidently a close family that rejoices in the glorious gifts two of their siblings possess.

Not to stereotype, but they bring an Oprah-esque sensibility to the refined art of watching tennis. They smile a lot and slightly overreact to some of the more outrageous shots their loved ones execute. And they are an unavoidable sight, with gold tresses and braids. Best of all is the father, Richard, daft and brilliant enough to dream this very future for his daughters and now merrily filming it all these years later.

After her experience in Paris, Serena probably needed this visual manifestation of support. Although the French crowd callously baited Martina Hingis in 1999, there was something more sinister and nasty in their treatment of the younger Williams player three weeks ago.

To suggest their booing and jeering was a politically motivated demonstration of anti-Americanism doesn't add up; why wait until the third set of a semi-final to make a political protest? No, it was more personal and while the thorny of issue of race was not broached, it was clear the French/Belgian crowd derived as much pleasure from what could have been construed as the humbling of Serena Williams as they did from Justine Henin-Hardenne's victory.

Since Venus led the charge upon the world game, complaints have emanated from the locker-room that the girls are aloof, that they hate to practice with anyone other than themselves and that they are arrogant. That the Williamses repeatedly refuse to categorise themselves as mere tennis players adds fuel to the furnace.

Far from being grateful for their status in the game, Venus admits she wants to serve her notions of fashion on the world of haute couture while Serena, a budding thespian, took time this week to lament the passing of Katherine Hepburn.

What that all suggests is that, in addition to spending God knows how many childhood hours on the baking West Coast tennis courts under their father's instruction, the Williams girls were at least allowed to develop as people. Now they are a sporting dynasty, the Williams girls' every movement can be construed as an act of defiance. They play the game but only on their terms.

Against Henin-Hardenne in the quarter-final this week at Wimbledon, one of Serena's outlandishly large gold-hooped ear-rings fell to the grass during play. The presence of such ostentatious jewellery irritated the female commentator, who reckoned she should cast the remaining one aside. Right on cue, Serena replaced the fallen accessory. Even though they are mannerly and soft-spoken in their interviews, the fearsome aggression and athleticism both sisters possess, their avant-garde image and that their ambition is unbreakable rubs people up the wrong way.

There is a masculine quality to Williams's game, as there was to that of Althea Gibson, the first black athlete to win Wimbledon in 1957. She paved the way under notoriously difficult circumstances. Growing up in Harlem, Gibson used to ride the subway all night rather than go home and confront her father, a violent drunk. She only entered the world of tennis when some local businessmen spotted her batting a ball around and decided to sponsor her.

Famously, she won Wimbledon the same summer as Arkansas national guardsmen blocked the path of nine black school kids from entering a school in Bill Clinton's home town of Little Rock. Across the Atlantic, a Harlem girl was meeting the Queen and dancing with Lew Hoad, the men's champion. Weirdly enough, the English crowd were indifferent to Gibson's colour but were left cold by the aggressive, overwhelming power of her game while America was, of course, obsessed with the colour of her skin but nonetheless determined to celebrate her championship.

Race is, of course, no longer presented in such stark terms. But the ascent of the Williams girls has brought it to the fore again. The Gibson era was a quirk. Female tennis remained a game for white girls.

The Williams girls have turned that on its head with such force and persuasion that people do not know quite how to react. George Vescey, the venerable American sportswriter observed that some British commentators appeared to see "intimations of Mike Tyson and Sonny Liston" in the Williams's style of game.

It is amazing those examples of brutal and notorious masculine strength should be used in the same breadth as two female exponents of a non-contact sport. Tennis is too polite to say it but for all the titles they are amassing, the Williams girls remain outsiders. That is partly probably down to their choosing. The truth is they do not need tennis, they are tennis. Imagine if they both quit after today.

Never in the history of sport has two members of one family been able to go out into their backyard and play a game of one-on-one - be it soccer, basketball, boxing, anything - knowing their game, their shared experience, is untouched for brilliance anywhere around the world. Essentially, that is what is happening in Wimbledon this afternoon. Maybe they should just move all the major titles to the Williams's home until the girls move on to other pursuits.

But theirs is an incredible story and only when they meet each other in these Grand Slam finals, only when one peerless talent bounces off the other instead of tearing asunder the inferior games of tennis's white cast, is it truly appreciated. That is why the arguments about which sister will be triumphant this afternoon are beside the point. They have already won. Modern tennis belongs to them and they deserve it.