“You’d be kind of hard pressed to find someone with that kind of tennis expertise that might not have interacted with the U.S.T.A. at some point,” Widmaier said.

Board members said that they had played no role in steering money to their own organizations and that in many cases the financial arrangements had predated their time on the board.

Gordon Smith, the executive director of the organization, defended its board as a group of impassioned advocates for the sport.

“A lot of people who are involved in the U.S.T.A. are willing to do things because they love the game, not because they’re trying to get something out of it or because they want to get some funding,” he said. “That’s not the issue.”

The U.S.T.A. is classified for tax purposes as a 501(c)6 organization, meaning it is not a charity but rather a nonprofit group, akin to a chamber of commerce or the N.F.L. That means the organization is largely exempt from paying federal taxes on about $213 million it generates in revenue, much of which comes from the United States Open, by far the group’s biggest event.

The U.S.T.A.’s national board sits above three national charities dedicated to goals like player development and helping disadvantaged athletes. It also oversees 17 regional chapters, as well as a lower level of state and local boards. Most of those organizations are set up as distinct nonprofit groups.

Image John Korff sat on a U.S.T.A. board that approved a $50,000 grant to Life Time Fitness, a health club chain. Months later, Life Time bought Korff’s company for an undisclosed sum. Credit... Paul Zimmerman/Getty Images

Experts on nonprofit groups said that the cozy connections at the U.S.T.A. raised concerns about conflicts of interest on the board, particularly because the organization had not clearly publicly disclosed any of the relationships.