For a decade, at least one of them has played every single Wimbledon final but one, and in eight of those ten years, a Williams has won. No one else has had a look-in.
The Williams know how to play the game off court as well as on, with perfect pace and preparation. They have played fewer events than any woman in the top 40 barring the three who have been off the Tour: Kim Clijsters, Justine Henin and Maria Sharapova.
The Williams march to their own tune, and ahead of Wimbledon it is no different: neither has played a single match on grass. Yet they seem destined to meet each other once again on finals day on second Saturday.
Are there any who can defy the odds?
In SerenaÃ¢â‚¬â„¢s half, her first interesting opponent comes in the shape of Sharapova in the fourth round. It is hard to believe that the statuesque Russian won Wimbledon way back in 2004, aged just 17Ã¢â‚¬â€after beating Serena.
With shoulder surgery behind her, she is showing signs of reaching something like her old form and made the final on BirminghamÃ¢â‚¬â„¢s grass last week. If she finds consistency, she may be ready to take Wimbledon by storm once more.
The quarters could throw up Agnieszka Radwanska who has reached that stage in the last two Wimbledons. She lost at Eastbourne to Victoria Azarenka, but if she is injury free she has the potential to shine on the grass.
In the semi-finals it could be Azarenka, though she too struggled with injury in reaching the Eastbourne finals. A more likely opponent at this stage is No.3 seed Caroline Wozniacki. The Danish teenager has improved with each of her three Wimbledons, though as defending champion, she lost in Eastbourne to Aravane Rezai.
The rising French star, Rezai, has raced up the rankings since last autumn, and won her first Premier title in Madrid a month ago. Her all-court game suits the grass, and she reached the semi-finals in Birmingham last week. However, she was forced to retire at Eastbourne, and that may play into the hands of another much-improved woman, Sam Stosur.
The strong Australian has used her fine doubles game to carve out some major singles successes, not least reaching the final of the French Open. She had some good wins at Eastbourne this week and, with her big forehand and heavy swinging serve, she may pose the biggest threat to Serena if she makes it to the semifinals.
And what about big sister, Venus? Well there are some interesting possibilities early on. She is unlikely to be troubled by the seeds in her eighth of the draw, but sitting amongst them is former world No.1 Ana Ivanovic
The Serb has been showing signs of a return from her tennis wilderness, taking some big scalps in Rome and fighting a tight battle against Jelena Jankovic in Madrid. She has ability on grass tooÃ¢â‚¬â€she was a semi-finalist at Wimbledon in 2007 and a fourth-rounder last year. The London grass could just stoke her confidence.
Venus could then face some classy and in-form serve-volleyers in the quarters. Francesca Schiavone, who turns 30 next week, is fresh from her biggest win at Roland Garros.
The Italian is riding a wave of confidence and have the perfect game for grass. They could meet in the fourth roundÃ¢â‚¬â€potentially one of the matches of WimbledonÃ¢â‚¬â€so Venus will have to face just one.
After that, the chances are that Venus will meet a Belgian for, like history repeating, Henin and Clijsters are drawn in the same segment. Both showed good form in different grass events this week, and Henin won in Holland.
She has made no secret of how much she wants to win Wimbledon, and she has the all-court game and varied stroke play to do so. If she beats Clijsters and then another Belgian, Yanina Wickmayer, she Ã¢â‚¬Å“onlyÃ¢â‚¬Â has to beat Venus and then her sister.
It is a tall order, but if she manages it, the moment would undoubtedly echo the one that inspired her to come back and try: Roger Federer at Roland Garros last year.
But the safe money? Venus to beat Serena in the final.
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BIOGRAPHY: Ethan Hazard
BIOGRAPHY: Daniel Sturridge