You might as well try to describe the colour of infinity as analyse a grand-slam final between the Williams sisters. Of all the remarkable elements connected to today’s showpiece, one of the most disconcerting is how Venus and Serena calmly handle the ultimate dilemma; to love and respect your sibling but want to defeat them while the world watches.

Would it not be all so much easier if they did not face each other? Apparently not.

“I definitely wanted her to win,” Venus, 29, said after her younger sister’s semi-final victory over Elena Dementieva, of Russia. “It’s like if she didn’t win or if I didn’t win, then the dream doesn’t come true that we’re both playing in the final. So I definitely want to play against her.”

Serena, 27, said: “The more we play, the better it gets. This is what we dreamt of when we were growing up. I wanted her to win and she wanted me to win. It all comes down to this.”

It has all come down to a particularly fascinating encounter. This will be the fourth time that they have met in a Wimbledon final, but, perhaps more significantly, neither has the edge. In 20 meetings, they have each won ten times, which in Serena’s words make today’s final “super-intense”. Venus said: “I guess it will be a tipping-point match. I do like to be ahead, even though she’s my sister.”

After their 6-1, 6-2 victory over Cara Black and Liezel Huber yesterday to reach the women’s doubles final — which they contest today, a few hours after their singles final — the Williams sisters were icily dismissive of questions about how their relationship alters as they prepare for meeting in a final. That might imply that they are having some difficulty coping, but in fact they are merely bored with the assumption that they struggle with the dilemma.

It was patently clear that their relationship does not need to be somehow put on hold — as the questions flooded in about facing a sibling, they would grin at each other and giggle at the same moment. But how does it work? How do they want the best for each other, to see the other win and then suddenly want to inflict a defeat?

Venus said it is “real easy” to separate the concepts of sister and rival “when you get a serve at about 127mph and it comes back as a winner. You soon realise you’re playing against an awesome player and you better really get ready. So that’s exactly when you separate it.”

The dilemma does exist, however, for Richard, their father, who will not be watching today.

“Serena didn’t play well,” he admitted after her three-set victory over Dementieva on Thursday. “She looked like a beginner, she was falling down half of the time. It takes nerves and guts to do what she did. It was the best match I’ve seen her play because she did not hit the ball well, but I didn’t enjoy it at all.

“Watching Venus’s match [against Dinara Safina] was more relaxing — and better for my heart, too. It’s the best that Venus has played consistently for a long time, but Venus is in a position to play better than she’s been playing.”

Venus said of her sister: “I know that she’s there for me . . . even if we’re 10,000 miles apart.” Today they will be on the same court and, albeit briefly, very apart.

Grand-slam grand dames

Venus Williams and Serena Williams have won a combined 17 grand-slam singles titles. Serena has lifted ten (four Australian Opens, three US Opens, two Wimbledons and one French title). She has 34 career singles titles. Venus has won five Wimbledons and two US Opens.

They have split the 20 matches between the two since they first played against each other in 1998. But Venus has won five of their past eight meetings.