As a boy, Gustavo Castillo ran hand in hand with his older brother from school to the American Tennis Club in the Andes Mountains in Bogota, Colombia, to chase errant balls from 3 p.m. till well after dark, helping to support five brothers and sisters after his father, a carpenter, died. This week, Castillo, 47, will be letting others chase the balls while he plays in the U.S. Professional Tennis Association championships in Boca Raton, Fla. The USPTA is an association of over 15,000 teaching professionals from around the world. Castillo, the pro at Argyle Country Club in Silver Spring, is ranked No. 5 in the U.S. in the men's 40-over division. Income was the primary motivation for the third-grader to shag balls, but being a ball boy in South America carries a proud legacy. "Most of the best tennis players in South America came up because they were ball boys," said Castillo. "You'd hardly ever see a son of a doctor or lawyer become a good player, I guess because rich people were thinking of their careers. Guillermo Vilas was the only one who'd never been a ball boy. His father was a doctor. "A person like myself had no other choice." The tennis came easy. Members soon paid young Castillo to hit with them. At age 17 he was the top junior in Colombia and ranked second on the men's tour. The following year he played on his country's Davis Cup team. "Being a ball boy was like going to school and learning to play tennis," he said. And tennis was a means of extricating himself from a future of manual labor. "If I had never learned to play tennis, I probably would be a mechanic or carpenter now," said Castillo. He joined his brothers in the United States in the early 1960s and played on the professional circuit in Florida. After getting married he started teaching and came to the Fountainhead Club in Hagerstown as teaching pro in 1966. He's been in Maryland ever since, working with Harold Solomon at Indian Springs and then with current women's Mid-Atlantic Tennis Association champion Carol Schindler at Argyle, where he's been for 10 years. Along with learning the game, Castillo also learned to construct tennis courts while in Colombia. He built the courts at Argyle and lately he's contemplated getting off the courts and concentrating solely on his tennis court construction company. But playing tennis and building courts go hand in hand. He gives many lessons on the courts behind his home in Potomac. "And that's how I get customers," he said. "I give them lessons and they come back and say they want a court built and I tell them I can build it." Anyway, he can't quit yet: "I still have plenty of juice in my legs and arms," he said.