There's no better proof of that axiom than Kerrie Brooks, who has served as the siblings' physical trainer for the past eight years. Brooks, a lithe brunette who looks as though she was separated at birth from French player Nathalie Dechy, became way more than just a traveling trainer for the Williamses -- she became a friend, confidante and as close as another sister. Brooks has been on board for the larger part of the Williams sisters' success -- Serena has won eight Grand Slam titles and Venus has won six majors.
So when Brooks and her husband, tennis photographer Ron Angle, decided to start a family, both sisters supported the decision. "They get it," said Brooks. Although it meant that Brooks would have to seriously cut back her travel schedule, the Williamses remained steadfastly loyal, choosing to work within Brooks' new restrictions. Venus and Serena are without a doubt surrogate aunts to Brooks' daughter, Addison, who turned 1 in April. (Serena was even at the hospital when she was born.)
I'm not going to be traveling as much as I have been," Brooks said. "I'm going to the French Open with them, but not to Wimbledon because my brother is getting married. And I'll probably go to the [U.S.] Open."
According to Brooks, the Williamses also have been overly supportive of her newest venture -- she recently established OrthoSport Group in Palm Beach Gardens, Fla., where non-celebrity athletes can avail themselves of her expertise in physical therapy and sports rehabilitation. Brooks, who has a doctorate in physical therapy from Shenandoah University and is a board-certified athletic trainer, will have regular office hours at the facility where she works with patients.
"It's always been a goal of mine to open my own facility," said Brooks, sneaking a bite of her turkey sandwich at Panera Bread before dashing off to the International Tennis Academy in Delray Beach, Fla., where she serves as the head of sports medicine. "I had a baby a year ago, and travel was getting to be a little bit harder."
The Williams, Williams and Brooks trio initially was put together by Kathleen Stroia, the vice president of sport sciences and medicine for the WTA Tour. Stroia had hoped to bring Brooks into the WTA fold, but at the time there was no permanent budget to do so. Though Brooks has worked with other tennis players as well as some golfers on the PGA and LPGA tours, she forged a special bond with the Williams sisters.
Brooks, who is a Palm Beach Gardens neighbor of the sisters, still works on the rehabilitation aspects of her famous charges' careers and is a constant voice of expertise on their off-court training. But she performs that role predominantly from home. This year, however, she will take her maternal grandmother and head to Paris for a rare road trip. The French Open is a key event to have a personal trainer in tow, as the lengthy matches on the grinding clay courts can take a toll on a player's body.
As far as Brooks is concerned, Venus and Serena's decision not to replace her with a new trainer, who could be on the road whenever they traveled, really should not be a problem.
"We've been doing it for eight years, so they pretty much know what to do," Brooks said. "We'll do a little tweaking here and there. Sometimes we'll vary things a little bit depending on what maybe is going on at the time. With tennis, the schedule is so long, certain types of training are hard to schedule."
That said, skeptics might wonder whether the Williams sisters' attention to training details might wander, but Brooks believes they understand the importance of keeping up with their off-court fitness.
"They have to take on a lot more responsibility and motivation on their own," Brooks said. "They've done it. I think every year as you get older as a tennis player, you have to work harder than the year before because you don't recover as quickly as you did when you were 16. Injuries linger a little bit longer. They realize this, and every year they are more committed, even though it is hard to motivate yourself to do the work."
Brooks is not too surprised that both sisters have had to fight through their share of injuries during their careers -- Venus has battled a pulled stomach muscle, wrist and elbow problems, and Serena has been daunted by a chronic left knee injury that already has required surgery.
"They are so physical on the court, every match they give 150 percent and go after every ball," Brooks said as explanation for the injuries. "They don't pull back at all. If you play like that every match, it's tough, it's tough on your body. To be at their level player, it takes just as much work off the court as on the court. Besides for the fitness needs off the court, there's the rehabilitation -- the massage, the stretching, the strengthening.
"I think everyone has injuries, but theirs become a little bit more public because they're bigger names."
Anyone who follows tennis on a regular basis knows that both Venus and Serena have been criticized at times for not appearing dedicated to the game because of their outside interests. Brooks believes that's not the case. She thinks they're more interesting, well-rounded people who love playing tennis, and she applauds their desire to know the world beyond the confines of a rectangular tennis court.
Some of Brooks' best memories of eight years of traveling with Venus and Serena were exploring the places they went while on the road as opposed to doing what many players often do at tournaments -- eat, sleep, practice and play matches.
For Brooks, it is her good fortune to be thought of as a relative by Venus and Serena, because in her mind the Williamses are part of the Brooks brood, too.
"They feel like family," Brooks said. "I love those girls, I really do. They're great people. They have good hearts. They're fun. They always make me laugh. You know tennis can be very, very intense, and when they're not playing, they're really trying to enjoy themselves and not think about tennis at all. They're well rounded and talk about anything because they're interested in everything."
Sandra Harwitt is a freelance tennis writer for ESPN.com.