Billie Jean King and World Team Tennis are serving up their 40th season, a feat accomplished only by the big four leagues in the United States.

Co-founder King recalls the early years of the mid-1970s when Robert Kraft owned the Boston Lobsters and Jerry Buss owned the Los Angeles Strings. She was the No. 1 pick in the WTT draft for the Philadelphia Freedoms, and Martina Navratilova played for Boston.

"I just remember the first night when I ran out, it was at the Spectrum. Of course, it's been imploded now like most of the places I've played in," King said recently, laughing while recalling the legendary Philadelphia venue. "It was quite a thrill to see it actually come to fruition."

Starting Sunday through July 29, the newer faces of the coed WTT summer league will take to the courts in seven cities.

Serena Williams will play for the defending champion Washington Kastles for the first time since 2011. Venus Williams is scheduled to make a home appearance for the Kastles, who are vying for their fifth straight WTT title, in the nation's capital on July 14.

Serena will compete on the road July 20 against Philadelphia and at home on July 21.

Youngsters Eugenie Bouchard (Boston), Madison Keys (San Diego) and Coco Vandeweghe (Philadelphia) return this month, along with veterans John Isner (Springfield, Missouri), Andy Roddick (Austin), and the chest-bumping doubles specialists Bob and Mike Bryan (Sacramento).

The conference championships begin July 30, and the final for the Billie Jean King Trophy will be shown live on Aug. 2 on ESPN2.

Here are a few things to know about the 40th season of WTT, which includes a preview show on Tennis Channel on Sunday:

SERENA'S BACK: Williams will be fresh off Wimbledon, where she's aiming for her 21st major title, when she takes the court for WTT. "It's great to have the No. 1 player in the world and maybe the greatest-ever female player," King said. "Venus has played more years than Serena, but I'm very indebted to both to them. They've been very loyal."

OLD FAVES: The first WTT match was played on May 6, 1974, at the Spectrum. King led her Freedoms to the win over the Pittsburgh Triangles. Within a year, her good friend Elton John released a song called "Philadelphia Freedom," which became a worldwide hit. A Who's Who of tennis has competed over the years: Jimmy Connors, Chris Evert, John McEnroe, Navratilova, Rosie Casals, Roy Emerson, Virginia Wade, Rod Laver, Andre Agassi, Steffi Graf and Bjorn Borg.

MENTORS: Several young players started out in WTT — Sloane Stephens, Taylor Townsend, Roddick, Keys — to help their transition from the juniors to the pros. "They get incredible mentorship from the older players, they get to practice with them, immediate feedback from them and the coach," King said. "It's about team, it's not about you. I always tell players, 'To be a real superstar means you make everybody on your team better.' "

ACES: Venus Williams and Roddick joined the WTT ownership group, which includes the USTA, in 2013. "We've been very fortunate to be nimble and adjust," said WTT CEO/Commissioner Ilana Kloss, whose league helped introduce instant replay and on-court coaching to the pro ranks. "When we look back and think of 40 seasons, it makes us really proud. But at the same time, it makes us look to the future. Bring the fans and the players what they want."