The story of Venus and Serena Williams is arguably one of the more unlikely and certainly fascinating stories of modern sports — the rise to the top of the tennis world of two African-American sisters from Compton, California, under the guidance of a taskmaster father, Richard Williams, with no connection to the traditional power channels and corridors of the game. So a documentary on the young women would seem to be a slam dunk, to mix sports metapors.

Co-directed by Maiken Baird and Michelle Major, Venus and Serena bills itself as “an unfiltered look into the remarkable lives of the greatest sister act professional tennis has ever seen.” But, in charting the Williams’ lives on and off the tennis court over the course of 2011, it mostly connects in only incidental and glancing fashion.

Definitely, the patriarch Williams emerges as an intriguing, Joe Jackson-type character. Both hard-driving and protective of his girls, he seems to have had less of an interest in tennis per se than in merely transforming his kids into superstars. Sacrificing their childhoods to grueling and unorthodox practice sessions in order to make them tennis robots was merely a means to this end. But for every fascinating tidbit the movie reveals (a 78-page “manifesto” of their destiny written by him before the girls’ birth, plus the fact that Williams apparently had another entire family, out of wedlock, concurrent with his raising of Venus and Serena and their older sisters), Baird and Major seem to take a step or three back, afraid of upsetting the elder Williams and/or their putative subjects with too many direct questions about him.

The filmmakers have great access that affords them all sorts of amazing footage (apparently big karaoke fans, there’s a scene of Venus and Serena singing Extreme), and that works fine in flashes here and there. But even though it tracks chronologically, with family home video footage interwoven to give some sense of backstory, Venus and Serena doesn’t feel like it has a strong sense of purpose or clarity. There’s a healthy, strange roster of supporting voices that Baird and Major summon as interviewees to testify to the sisters’ sociocultural significance and general greatness — a group that includes Anna Wintour, Bill Clinton, Chris Rock, John McEnroe and author Guy Talese — but there’s little rhyme or reason to how their thoughts are integrated. In short, there’s a far more cogent and coherent film to be made about the importance of Venus and Serena Williams. This one is entertaining, and certainly hits the many highs and few lows of the sisters’ careers, but it’s safe, polite and timid.

Housed in a regular Blu-ray case, Venus and Serena comes to the format in a 1080p high definition 1.78:1 widescreen transfer, with a DTS-HD 5.1 master audio track. Under a full complement of chapter stops, the Blu-ray also includes a nice slate of bonus features, anchored by a clutch of deleted scenes and an AXS TV behind-the-scenes featurette. There are also interviews with co-directors Baird and Major. To purchase the Blu-ray via Half, click here; to purchase it via Amazon, click here. C+ (Movie) B (Disc)