And finally tonight, venus and serena williams, a new documentary takes us into their private lives, where we see the discipline and sacrifice it takes to stay on top. Even when you're serena, now 31,... See More
And finally tonight, venus and serena williams, a new documentary takes us into their private lives, where we see the discipline and sacrifice it takes to stay on top. Even when you're serena, now 31, and the oldest number one ranked female player in history. And the two are proving age is no match for the heart of a champion. Here's abc's linsey davis. Reporter: Riddled at times with doubt, fatigue, and pain. I'm tired. It's so hard today. Reporter: This is what it looks like to be a champion. I feel like I haven't accomplished anything in my career, and maybe that sounds ridiculous, but I feel like i have to do so much more. Reporter: For more than 20 years, day in and day out, five hours of exhausting drills, running ten times around a football field. Can't wait to get stronger. Reporter: In a new documentary, venus and serena, we get a glimpse of the williams' super human discipline. Ahh. Reporter: Way before their combined net worth was estimated at more than $150 million, when they were barely tall enough to see over the nets on the tennis courts in compton, this was all they knew. What's the toughest match you've ever been in? Probably the one against my sister. Reporter: Their dad, the man who borrowed used balls from country clubs to coach his daughters in a sport he never played in a park infamous for gang activity. I sat down with the film makers. Earl woods, joe jackson -- does ard williams belong in tt same category? Richard williams doesn't belong in that category at all because he raised them with a great deal of love, and he also always gave them a sense of freedom. He's fiercely protective of them. Reporter: And he taught them to be protective of each other. They are both jehovah's witnesses, both unmarried. They not only share the same house in florida -- no, I don't want to move out. Why would I want to move out? Reporter: -- They share some of the same hobbies. ♪ some of the same doubts. I have a hard time doing the things I'm supposed to do. Reporter: Both know the burden of keeping a promise they made as little girls to the people of compton and beyond. You gonna win for us? Mhmm, good girl. Reporter: It is a story of triumph over adversity for a family that mastered a game with a scoring system that always begins with love. Linsey davis, abc news, new york city. And the documentary venus & conservativeena opens in theaters may 10.
This transcript has been automatically generated and may not be 100% accurate.