In this April 30, 2008 file photo, Billie Jean King laughs at a Washington Kastles function. (Jacquelyn Martin/AP)

Of all the things icon Billie Jean King has accomplished in tennis, helping found the World Team Tennis league is at the top of her list. It’s more important to her than starting the Women’s Tennis Association, and winning 39 Grand Slam singles, doubles and mixed doubles titles.

“Nobody believes it,” King said.

World Team Tennis is in its 39th year, and King, a co-founder, said there’s room for the league to expand, both nationally and internationally. But the league struggles for significance, with few American champions in the league and limited television opportunities.

“We’re small,” King said. “Unfortunately, I’m from the wrong generation. If I had the money myself, we’d be in probably a little different position.”

At least one match is streamed on ESPN3 every night of the three-week season, but the league has to pay thousands for each match ESPN streams online. King said she wants to stream every match next year, even if it’s somewhere other than ESPN3.

Thinking back to when she was playing in the 1970s, King reminisced about tennis being so popular then that getting time on a recreational court was a challenge. Now, she has to rack her brain to think of any American champions in the league other than the Williams sisters. Prominent players have to balance their tournament schedules with rest when it comes to joining a team for a few matches.

King said the World Team Tennis format — men and women on the same court — is followed on the collegiate club level, but not on the varsity level.

“You know March Madness? That’s what we could be,” King said.

World Team Tennis has scaled back from 16 teams to its current seven franchises. King said she ideally would have eight teams, but she said she has had talks with other countries about taking the league there.

“We will expand when we’re ready to expand and we do it right,” King said. “Otherwise we should not.”

Though team owner Mark Ein feels the Kastles are established in Washington after four championships in the past five years, he acknowledged that in the District and on the national stage, many don’t know World Team Tennis exists. Players agree.

“I think World Team Tennis is really underrated,” the Philadelphia Freedom’s Taylor Townsend said. “It should have more recognition than it does. Unfortunately, people watch the Slams, and the Slams are televised all of the time. These aren’t.”

Ein said the Kastles sold out their first match ever in 2008, partially because Serena Williams played that night but also because the organization took a year leading up to the opener to market World Team Tennis. The Kastles partnered with local tennis professionals and tennis clubs, showing video of what a World Team Tennis match would look like. The focus now is on social media.

King said she doubts any of her dreams for sweeping changes in tennis will occur in her lifetime, but the team tennis format is enough to bring a smile to her face.

“It just reflects the way I want the world to look: men and women together,” King said. “We’re all in this world together.”