What makes a great tennis player? How can one tell when a talented teenager will blossom into a top professional? How much of the game is mental rather than physical? Is success really dictated by how much a player “wants it,” as current and former great players are wont to say? These basic questions have no answers, at least none not rendered meaningless by all the “ifs” and “buts” that come along with them. Those who scout athletic talent have systems, superstitions, and unshakeable beliefs, but in the end, they are not much different from those of us who buy lottery tickets — brimming with hope despite very, very long odds.

“Unstrung,” an excellent documentary featured at the Tribeca Film Festival, offers all the evidence of this we need. The film, directed by Robert Klug and co-produced by former world no.1 Jim Courier, presents seven players, all of them young boys preparing for the 2005 USTA National Championships, held each summer in Kalamazoo, Mich. For junior tennis players, no tournament is more important than the one in Kalamazoo. The winner can call himself the best junior in the country, and he receives a wild card into that year’s U.S. Open. There are other perks: calls from agents, marketing deals, publicity, et cetera.

Klug’s subjects are diverse in background and personality, and only two of them, Donald Young and Sam Querrey, are recognizable figures (and even then, only to tennis fans). Marcus Fugate, a muscular 6-foot-2-inch boy from Rochester, N.Y., turns the heads of young women at Nick Bollettieri’s tennis academy in Florida. Clancy Shields, from Idaho, travels from tournament to tournament in a van with his father. Tim Neilly of the Bahamas tries to land sponsorship deals over the telephone while his mother, who is suffering from cancer by the end of the film, advises him. Gregory Hirshman of La Jolla, Calif., enjoys his Rubik’s Cube, mathematics, and the violin as much as his tennis. The only character who resembles the stereotypical, racket-throwing tennis brat is Holden Seguso, son of former doubles great Robert Seguso and Carling Bassett, a former top 10 player from Canada who mixed tennis, modeling, and acting long before anyone had heard of Anna Kournikova.

If Klug were a scout, he would be deemed a failure. He started with a dozen or so players and narrowed the field to seven for the film, and yet the best player of the bunch, Querrey, does not appear until late. Even when Querrey loses to Young in the final of the 2005 tournament, there’s little indication that in the next two years he will vault past his peers and into the top 100 on the men’s tour. At the outset, Fugate is presented as the most talented — largely because coaches, like teenage girls, marvel at his physique. Seguso has the big forehand and the pedigree; if only he would lose the attitude. Young, the top-ranked player despite being age 15, is given star status.

The real star of the film, however, and the one who teaches us the most about the sport and the vagaries of talent spotting, is Hirshman, who was in the audience with his family during Saturday’s showing.

Hirshman talks to himself on court, though he never utters anything obscene or breaks a racket. Off court, he fills a binder with 600 pages of mathematical calculations and talks about chemistry with his father, Paul. He practices Mozart. At home in California, he volunteers as an area leader for the Republican Party. He writes acrostic poetry; one of his poems, “Children,” has been published.

Hirshman’s mother, Barbara, suffers from injuries she sustained when she and her husband were hit by a car while riding their bikes. When Paul tells the story of Barbara’s learning to walk again, it becomes clear why tennis is just a game for the Hirshman family. Paul cares more that Gregory doesn’t become a “mean person” than if he wins tournaments (during one of Gregory’s matches, Paul studies Spanish to keep himself from watching; after his son loses, he isn’t the least bit disappointed).

Gregory, who doesn’t look athletic and has imperfect strokes, is not the last person you would figure for a top junior tennis player, but if he were placed in a lineup alongside the rest of his co-stars, he probably would have received the least votes. That Hirshman found his way into the junior elite and will play for Stanford next year is what’s great about this film, and about tennis — it’s a sport that accepts all kinds. Yes, everyone who plays must run, think, swing at a moving object, and recognize an opponent’s weaknesses, but no one can say that any one of these skills is absolutely more important than another. Not even hard work decides things, despite the proclamations of several coaches and former stars who appear in the film. Plenty of great pros survived while others who trained more did not. It’s the combination of these things that matters, and identifying whether a person has the proper chemistry for success is impossible.

Kalamazoo offers us the ultimate proof of this. Stan Smith won the 18-and-under junior event in 1964. Since then, 39 boys have won 42 championships (Paul Goldstein, Phillip King, and Young each won it twice). Of those 39 players, one — Michael Chang — won a Grand Slam title. If you have a chance to see this film — it has two more screenings this week (see www.tribecafilmfestival.org for details) — ask yourself, who would have predicted that?

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Earlier this year, when Mats Wilander was asked about the prospects of Rafael Nadal equaling his performance of 2006 by winning every clay-court match, the three-time French Open champion from Sweden said, “I don’t see that happening.” So far, Nadal sees it differently. The 20-year-old Spaniard won his 72nd consecutive match on clay yesterday to take his third straight title in Barcelona, 6–3, 6–4 over Guillermo Canas. He’ll try to defend his title in Rome beginning next week. If he does, don’t be surprised if he takes a pass on Hamburg so he can rest for a run at his third French Open title.

tperrotta@nysun.com