Serena and Venus Williams share a house back home in Palm Beach Gardens and are renting accommodation together here.
They practise together, play doubles together and, when they are in opposite halves of the draw, frequently face each other across the net in finals. Their meeting in today's Wimbledon singles final will be the fourth occasion they have faced one another for the right to lift the Venus Rosewater Dish.
In the early days there were suggestions, always flatly denied, that the sisters shared victories along with everything else. Some of their early meetings were indeed strangely subdued affairs, with both women apparently playing below their best.
Was that really surprising? It would surely be a surreal experience for any pair of sisters or brothers, let alone two Americans brought up on the public courts of Compton in California, to find themselves playing one another in front of packed stadiums and television viewers worldwide.
As the years have gone by and the experience of playing each other has become almost routine, so the quality of their matches has risen sharply. Last year's Wimbledon final was one of the best for years, Venus wresting the initiative back after her sister had made a flying start.
It was bettered by their meeting in the quarter-finals of last year's US Open, which in the eyes of many was the women's match of the year. It was an awesome display of power, aggression, skill, athleticism and competitive spirit. Serena won and went on to claim her first US Open title for six years.
Yesterday the sisters joined forces to defeat Cara Black and Liezel Huber 6-1, 6-2 to reach the doubles final, but Venus promised a full-blooded battle when they meet on opposite sides of the net today.
"I'm happy for her to be in the final but I have to face her and defeat her," Venus said. "I don't necessarily want her to lose, but for sure I want me to win. Maybe that doesn't make sense but, when I'm playing someone else, for sure I want them to lose.
"I don't like to ever see her disappointed in any way. But at the same time I don't want to see myself disappointed. I need to get my titles, too. I definitely wanted to play against her because the dream has come true for both of us and for our family."
If they can recreate the form of their first six matches here, the sisters could stage another epic. Venus in particular has been in awesome form. In destroying Dinara Safina, the world No 1, for the loss of only one game in the semi-finals, she extended her record to 34 winning sets in a row on these courts. She has not been beaten here since Jelena Jankovic got the better of her in the third round three years ago.
Her semi-final could hardly have been a greater contrast to Serena's. The younger sister was detained on court for 11 minutes short of three hours by Elena Dementieva, the Olympic champion. However, Serena had been just as impressive en route to the last four, having won all her previous matches in straight sets and never having to play for more than an hour and a quarter.
While Serena has the more effective serve – even if her sister can hit the ball harder – it is Venus who has the better game for grass. She is confident at the net, where her huge reach makes it very hard for opponents to pass her. Serena, in contrast, never looks comfortable on her volleys and is at her best when slugging it out from the baseline. If Venus has the best backhand in women's tennis, it would be hard to come up with a better forehand than Serena's.
Both players are wonderful competitors, but Serena's victory over Dementieva was further proof that there is no greater fighter in the women's game. "Even if she's not playing her best, just that fight she has, you're facing that," Venus said. "There's so much to face when you play her. It's definitely a lot to get your mind around." The sisters stand at 10-10 in their head-to-head record. For three years they did not play each other, but since resuming battle last year they have shared the spoils, each winning three times. This year Venus won in three sets in Dubai and Serena in three sets in Miami.
Whoever wins, Richard Williams, their father, will not be there to see it. He will be on a flight back home, his work done, having guided his daughters to yet another final. "Lots of times I've had to tell the captain of the airplane not to tell me the score but last year he came and told me," Richard said. "This time I'm flying a different airplane and I'm going to say to him: 'Please don't tell me'."
Just Williams: Sisters' three previous finals
2002: Serena won 7-6, 6-3
Previous matches between the two sisters had been disappointing, but few could complain about their first Wimbledon final. Venus was unbeaten in 20 matches at the All England Club going into the final but she had a sore arm which restricted her serve and it was soon clear that her sister would provide a major test. Serena, who had won her previous three matches against Venus without dropping a single set, won the first set tie-break 7-4 with an ace and took the second set with something to spare.
2003: Serena won 4-6, 6-4, 6-2
Serena's subdued celebrations as she walked to the net and gave her sister a consoling hug summed up a disappointing final. Venus was never in shape to give of her best, having gone into the match suffering from a damaged stomach muscle and groin and thigh strains. She would almost certainly not have played had this been anything other than a Grand Slam final. The match featured plenty of impressive shot-making, but in the end Serena was a comparatively easy winner.
2008: Venus won 7-5, 6-4
Venus trailed 0-2 and 0-30 before finally putting her game together. The defending champion soon took charge, though the match remained competitive throughout. Serving at 5-6 and 30-30, Serena bowed to pressure from a barrage of ground strokes and put two backhands in the net. Serena broke in the third game of the second set, but Venus hit back immediately. At 4-5 and 15-40 Serena saved the first match point with her ninth ace but then put a backhand out to give Venus the title.