Strapping 17-year-old Tommy Lee of Snellville hunches along the baseline like a sprinter in the starting blocks, waiting for a tennis ball to roll across the court.

“Go!” commands Patricia Jensen as the ball is let loose. Lee bolts forward, takes a half-dozen furious strides and screeches to a stop. He corrals the ball in a single, fluid swipe, turns and ... slips, falling hard. The 19 other teenagers watching from the sideline cringe in empathy: There, but for the grace of their sneaker soles, go they.

It’s a sunny summer afternoon and their friends are probably at the pool or a mall, but Lee and the others are here under the dim light of the indoor tennis facility at the Racquet Club of the South in Norcross, being tested for their speed, agility and tennis aptitude.

They are trying out to be ball persons at the upcoming Atlanta Tennis Championships, a men’s professional event that runs July 18-24. As evidenced by Lee’s scraped and bloodied elbow, it’s a job that’s harder than it might look.

Not that anyone really looks. Ball persons (girls are a regular part of the scene now) perform in a curious substrata of the sports world. They are unpaid volunteer servants to the stars who must do their job discreetly, keep their thoughts to themselves and get off the stage fast.

If you think baseball grounds crews have it rough, watch a ball person at work. They toil in temperatures that can hover around 100 degrees and they’re required to suck it up whenever a cranky pro demands another ball or towel or water.

Ball persons also have a good bit to remember. The ball in-play/ball out-of-play scenario that is repeated again and again during a match creates a rather elaborate silent choreography in which things can easily go awry. It’s not hard for a ball person to blunder into a live ball, especially under duress. With the latter portion of the tournament televised, one could screw up on national TV.

The trade-off: a chance to be part of the action.

“It sounds like a blast,” Lee explained. “I think it would be cool to be on TV and to be around the players on the court.”

The woman running the tryout is Jensen, mom of former tennis pros Luke and Murphy Jensen.

Jensen’s manner balances between chops-busting coach and protective den mother. When one boy jogs to get a ball, she yells out, “You’re running like a girl. What are you? A cheerleader?”

To a girl who can’t manage to throw the ball from one baseline to the other without hitting the net, she advises, “Step into it.”

The essence of the job, Jensen said, is, “Service all the needs of players on the court and expedite the match.”

There are rules to follow, depending on where the ball person is stationed. For those at the net, for example, it’s essential to always run forward to the opposite sideline after picking up a dead ball.

Jensen tells the group this repeatedly. Still, one candidate grabs the ball, turns and retreats to his starting spot. He does this twice before getting it right.

Some in the group are nervous, but Jensen breaks the mood with frequent bits of humor. One of her foils is 39-year-old Dunwoody resident Tony Martin who is assisting with the tryouts but who originally came to the club with the idea of becoming a ball person himself.

“I definitely would have been out of place — by about 25 years,” Martin said. “I didn’t think it would be by that much.”

For this reason, he has been dubbed “Kramer,” in honor of the “Seinfeld” character who in a memorable TV episode tried out and became a ball person at the U.S. Open, to the annoyance of the younger candidates.

Kramer, of course, fouled up and created a major scene.

Tournament director Bob Bryant said it’s not unusual in a tournament for a ball person to, “mess something up a little bit.” But, he added, “They usually settle into it. Most of [those trying out] are really into tennis and it’s a great opportunity to be up close to a professional event.”

The facts

What: Atlanta Tennis Championships

When: Main draw: July 18-24. Qualifying: Saturday-July 17.

Where: Racquet Club of the South, Norcross.

Who: Tennis pros including Mardy Fish, Lleyton Hewitt, John Isner, James Blake.

Prize money: $531,000.

Attendance in 2010: 41,000