Three years before the Battle of the Sexes, WTA founder Billie Jean King was one of nine courageous women out to take stand for equal rights in tennis.

The Amateur Era had come to an end, and though Open tennis had just begun, the troubles were hardly over for even the top women in the game.

"I didn't have any idea we were going to get different prize money," King said in an interview for the PBS documentary American Masters. King won the first Open Era Wimbledon Championships alongside Rod Laver, who earned £2000 in prize money to King's £750. "I thought it was totally unfair."

Disparate prize money was only the tip of the iceberg. As the men's game continued to grow, fewer women's events were being held alongside them, leaving fewer opportunities for women to succeed as professional tennis players.

"We were in big trouble if we wanted to keep playing tennis."

King enlisted the help of Rosie Casals and Nancy Richey, who accompanied her to speak with Gladys Heldman, publisher of World Tennis Magazine and mother to pro Julie Heldman. They wanted to boycott an event to be held after the 1970 US Open, one that leveled prize money 12:1 against the women competing.

Heldman decided to go beyond passive resistence and urged the women to start a tour of their own; a tournament was organized in Texas.

"She knew how to get a sponsor, she had great connections," said Valerie Ziegenfuss, one of the nine players who signed up for the Houston Women's Invitational.

Looking to stop the revolution that was quickly unfolding, the USTA - then known as the United States Lawn Tennis Association - threatened to suspend anyone who planned to play this one-off event. Many of the top players stayed away, but nine women were undeterred.

King, Casals, Richey, Heldman, and Ziegenfuss were joined by Judy Dalton, Kerry Melville Reid, Peaches Bartkowicz, and Kristy Pigeon, all of whom signed onto what would become the Virginia Slims Circuit for one dollar.

"We weren't sure about our destiny but we knew it was in our hands for the first time," King recalled.

Casals went on to win the tournament in Houston, but women everywhere won a whole lot more. The Virginia Slims Circuit became the Women's Tennis Association in 1973, and the world of women's sports would never be the same.

Six members of the Original Nine recently reunited at this year's US Open; check out that story here.