Very few professional athletes have dominated their sport as authoritatively as sisters Venus and Serena Williams ruled over women's tennis during their heyday. Both as individual competitors and as the most successful doubles team of their generation, they amassed a record number of Grand Slam and Olympic titles. Maiken Baird and Michelle Major's documentary about the duo certainly celebrates their remarkable achievements, but it only hints at the darker aspects of their personal story. The film crew had access to the pair during the 2011 tennis season, which turned out to be a year when the sisters hardly played at all due to various injuries. The majority of the movie's interest comes not from their rehabbing, but from archival footage of them as youngsters, practicing for hours and hours under the guidance of their father Richard Williams, who -- with his outspoken and gruff demeanor -- became one of the more controversial tennis parents (a subculture known for its often problematic members) of his time. While their international success makes Venus and Serena rich material for a documentary, it's hardly the only fascinating aspect of their lives. That becomes apparent during the latter half of the movie when, having established how they got where they are now, it tentatively explores the psychological makeup of the sisters. Venus, the elder of the two, seems to retreat from the film, in large part because she comes off as a stable person with interests beyond tennis. During interviews throughout the picture you realize that she doesn't define herself by her sports career. Serena, on the other hand, appears ready for her own reality series. The younger sister revels in discussing a half-dozen or so personalities she not only has, but has named -- their mother and other people in their circle also call these alter egos by their respective names. Serena is asked about her occasional on-court tantrums, including an infamous U.S. Open match in which, after being called for a foot fault, she physically threatened a line judge. Those who know her blame it on one of her other personalities, but the filmmakers attempt to put the incident in context by showing profanity-laced tirades from greats like John McEnroe and Jimmy Connors, and serving up talking heads saying that she was likely criticized so strongly for this incident because she was a woman. That argument ignores the fact that in those older clips, none of the guys talk about shoving a ball down an official's throat. That moment -- one for which she refused to apologize for following the match, but did offer a mea culpa three days later after being fined -- becomes more telling later in the film when their father, making a joke about how annoying his first wife was, says that too much of anything will drive you crazy. That's an easy laugh line when talking about a relationship, but when you think about how we've seen Richard make his daughters practice repeatedly in their youth, how he worked them and worked them in order to produce a pair of moneymaking machines (they all readily admit he doesn't much like tennis), you want the filmmakers to ask tougher questions of Serena. She's presented as the classic younger sibling who is hyper-competitive with Venus, something big sis notes on camera and may explain why Serena has more wins when they've played each other head-to-head. Yet Baird and Major seem wary of digging too deeply into her psyche. At one point, one of the Williams sisters declares that they don't relate to perfectionists, but they completely understand obsessives. If Baird and Major had tried to get them to be more introspective, the movie would have been a fuller portrait of what someone has to sacrifice to become the best in the world. Considering their climb out of poverty and the racial hostitily they faced becoming the undisputed champions in a sport still dominated by white players, Venus and Serena's story is infinitely inspiring. That alone makes the movie a rock-solid documentary that will appeal to you regardless of your interest in tennis. However, it does leave you with a bunch of questions that you wish the first-time directors had worked up the courage to ask.