Time, I think, to start marvelling at those Williams girls. They really are the most remarkable pair in the history of tennis. We have known them as child prodigies, we have known them as youthful champions, we have known them as supreme champions at their absolute peak. And now they are giving us something new: they are giving us longevity. If things go on much longer, people will start to love them.

Perhaps we should start. Certainly, we should work up a little admiration. Yesterday, Serena Williams joined her sister, Venus, in the final after what was eventually a perfectly splendid semi-final. She beat Zheng Jie, of China, 6-2, 7-6. Zheng is ranked No133 to Williams's 6, she was giving away 4 inches in height and a couple of stone in weight.

And if we hadn't had rain delays, we might have had a shock, because Zheng was moving Williams all over the court, hitting hard and low and flat and finding the corners. Williams doesn't care too much for the running about part of tennis - she is better at standing still and whacking. She was beginning to look flustered and also a bit blown.

But the rain came and she got her breath back, and though she was taken to a tie-break in the second set, she served it out so ferociously that Zheng was blasted aside. The match ended on a double fault, an unworthy way for Zheng to lose. She has been one of the great joys of this Wimbledon and her sharp-angled, two-fisted cross-courter is the most thrilling shot we have seen in the women's competition.

All the same, it was Williams who won, and it is Williams whom she meets in the final, which is a cue for the inevitable groans and cynical suggestions that they will decide who wins over breakfast tomorrow. Let us not do unworthy things ourselves, let us instead concentrate on the splendour that is Williams.

I first encountered them as pig-tailed, little ball-whackers at Nick Bollettieri's tennis factory in Florida. They were already baby stars. Bollettieri told them how to bash clay dust from their shoes and coached them with his usual breathless excitement while a lanky fellow filmed every move for some mad purpose of his own. This was, of course, the much-mocked dad, Richard Williams. He should be celebrated as the tennis genius of our age.

The legend of the sisters' upbringing is not exaggerated: Compton in Los Angeles really is not the place for a pleasant evening stroll. That one world-class player should emerge from the cracked public tennis courts there is remarkable, that two should do so, and from the same family, is little short of a miracle.

True, they have been a little bit abrasive at times, sometimes a bit awkward, sometimes a little rough about the edges. Well, that's often the way with mould-breakers. You cannot always change society with supreme tact and patrician manners. Sometimes the sisters have seemed to treat the conventions of the game with contempt, winning the warm-up, overdoing the glares, not apologising for points won from a net-cord, getting too close at the change of ends. Little things, niggly things that add up.

This made them seem a little too strident, a little too in-yer-face. But how could they have been anything else? They needed to batter down walls, they needed, most importantly, to feel that they were dealing with this strange and somewhat hostile world on their own terms. And so they did and if people didn't like it, then that was their rotten luck.

Their tennis is pretty charmless, if pretty devastating. Each has a game that inspires admiration, rather than affection. Both play with little more than power, they like to blast everything in their way. They seem to have a taste not just for winning but for humiliating an opponent. Perhaps that is just a by-product of their style, perhaps it is their nature. It is impossible to say, only to notice that opponents defeated by the Williams sisters do seem to be unusually well beaten.

The sisters lack the sense of vulnerability that attends almost all tennis players, of either sex, even the best. That's because, uniquely, they are not alone. There are two of them, tight, loved and loving, and that is a source of extraordinary strength to them. And they go on and they go on, and here they are at Wimbledon and one of them is going to win it again.

Tennis has never lost its charm for them, largely because their father helped them to cultivate interests outside the game. This is the secret of their longevity, the Williamses have been winning here since 2000 and tomorrow for the seventh time, a Williams will win again.

Whether they sort it out over coffee and muffins or whether they will get tied up in the complexities of their relationship, the chances are that the actual match will be a bit of a damp squib.

Don't blame them, instead, celebrate the extraordinary nature of their achievement in getting here, again, the third time the two of them have contested a Wimbledon final.

Whoever wins, it is a victory for the Williamses. These days, they have mellowed and cut down on some of the snarling, but they are still prepared to fight anybody. Absolutely anybody. Except each other, of course.