"the spirit of humility, sacrifice, willpower and responsibility."

Is it the city's renowned cheese and sausage? Its clean air? Or does the credit for Tandil's five world-class pro tennis players go mainly to no-nonsense coach Marcelo Gomez?

Whichever the case, Juan Martin del Potro's recent U.S. Open victory has cast a spotlight on the medium-sized Argentine dairy town on the Pampas and has the tennis world pondering just what Tandil has going for it to produce so much athletic talent.

The 39-year-old Gomez trained the lanky Del Potro from the time he was barely able to hold a racket, and four other players currently on the men's tour. For a town of 130,000, the number of tennis stars is unusually high.

Del Potro's victory over Roger Federer at last month's tournament in New York has made Gomez's Independent Club a popular destination for journalists and tourists, all wondering the same thing.

"People ask what it is about this place, and I don't have an answer. I think they should commission a scientific study to find out," says a smiling Gomez during an interview at the club.

Such a study would almost certainly focus on the tightknit community's rabid interest in sports, including soccer, rugby, field hockey and tennis, usually in that order. It might also elaborate on the work ethic of the city's largely Italian, Spanish and Danish immigrant population.

It would note that virtually every shop in the quiet, granite-paved downtown seems to have posters of Del Potro and other home-grown sports stars, including Mauro Camoranesi, who plays in the Italian soccer league.

It might also highlight the tranquillity of a community 200 miles south of Buenos Aires, one that is surrounded by undulating hills, which attract weekend hikers and cyclists.

But in the end, this town's successful development of pro tennis players is most often viewed as testimony to the youth program led by Gomez, a coach who found his niche as an 18-year-old college student looking for a way to finance his studies.

The tall, dark-featured native of Cordoba state, whose sun-dried skin tells of countless hours on the tennis court -- including 13 years training "Delpo," as the 21-year-old Del Potro is known here -- rules over a highly structured training program at the semi-private Independent Club and its eight clay courts.

Del Potro, currently ranked fifth in the world, first picked up a racket at age 6 under Gomez's tutelage. So did Juan Monaco (ranked No. 31) Maximo Gonzalez (68) and Diego Junqueira (165). In 2000, Gomez's student Mariano Zabaleta reached a world ranking of No. 21, but has since slipped to 357.

Gomez describes himself as "energetic and serious, but not a shouter," who runs a "militaristic" program.

A pro since age 17, Del Potro embodies the Gomez style of aggressive shot-making, of "giving everything you've got to the ball, hitting as hard as you can," says the coach.

But Gomez says his training regimen also focuses on the mental aspects of the game.

"People consider me firm and clear in the ideas that I use with a player. I emphasize the spirit of humility, sacrifice, willpower and responsibility."

Above Marcelo Gomez (C), who coached Argentina's Juan Martin del Potro since he was a child until 2007, is carried by friends as they celebrate Del Potro's victory against Switzerland's Roger Federer in the men's singles finals match at the U.S. Open tennis championship, in his hometown of Tandil, September 14, 2009.