“Because of him, we became ‘enemies’ and had to move to Serbia,” Pavlov said in an email. “My father appealed for peace on TV.”

With the rest of Kecmanovic’s family back in Serbia, Pavlov has provided vital support during Kecmanovic’s transition to the United States. In 2013, when the family first debated the offer from IMG, one of the world’s most famous sports academies, she volunteered to go with Kecmanovic as his guardian. Otherwise, his parents would not have allowed their son to go alone.

“It was very hard, but that was his destiny, not mine,” Kecmanovic’s father, Dragutin, said in a telephone interview from his home in Belgrade. “We missed him very much. You go into your room and cry a little bit and then come out and everything is O.K.”

Pavlov has a Ph.D. in psychology, and her research on entrepreneurs who live and work in two countries helped prepare her for the adventure with her nephew. Because she was not married and had a job that allowed her to work from home, she was the obvious candidate to accompany Kecmanovic to Florida. But everyone in the family had a role.

Kecmanovic’s mother was an early coach, focusing on his mental discipline. His father helped decide which tournaments to play in and did much of the driving. A grandmother was in charge of his education and cooked for him, and a grandfather would take him to tournaments when his parents were too busy to attend.

But it fell to Pavlov to handle virtually all of those duties once they went to Florida, while also continuing her work online for Group 484, an organization that provides support for migrants in Serbia and Macedonia. In the beginning she could barely cook, had difficulty with English and did not own a car.