Former first lady Michelle Obama spoke out Monday in support of equal pay in womens sports, becoming the latest in a growing group of prominent voices pushing to close the salary gap between male and female athletes.

Speaking on the 50th anniversary of equal prize money being awarded at the U.S. Open, Obama told the crowd at Arthur Ashe Stadium that there are far too many tournaments that still need to give equal pay to women.

Obama also honored Billie Jean King , the 79-year-old tennis legend who long advocated for the United States Tennis Association and sponsors to facilitate equal prize money. The 1972 U.S. Open champion united her fellow female athletes to threaten a boycott of the following years tournament unless women received the same pay as men.

In 1973, the U.S. Open became the first sporting event to offer equal prize money $25,000 to both male and female players. It would take 34 years before all the other Grand Slam tournaments followed suit. This year, singles champions will earn $3 million.

Billie Jean had a choice. She could put her head down, keep winning tournament after tournament and just accept whatever check she was given, Obama said. Or she could dig deep and break serve, she could make a stand.