Barack and Michelle Obama were in the house at the U.S. Open on opening night to watch and support Coco Gauff, and the former First Lady also used the occasion to advocate for equal pay for women in sports.

After Gauff’s three-set victory, Michelle Obama spoke as part of a tribute to Billie Jean King marking the 50th anniversary of the U.S. Open becoming the first Grand Slam tennis tournament to pay equal prize money to women.

“Billie Jean teaches us that when things lie in the balance, we all have a choice to make,” Michelle Obama said. “We can either wait around and accept what we’re given. We can sit silently and hope someone else fights our battles. Or we can make our own stand.

“We can use whatever platforms we have to speak out and fight to protect the progress we’ve made, and level the playing field for all of our daughters and their daughters.”

When King won the U.S. Open ion 1972, she earned $10,000 for her title, $15,000 less than what men’s champion Ilie Nastase made. She threatened to not play at all the next year — and added that no other women would, either.

King recruited a sponsor, Bristol Myers Squibb BMY , that stepped in and helped make up the difference in 1973. That year, the two singles champions were paid the same amount: $25,000. It wasn’t until more than 30 years later that Wimbledon became the last major tennis tournament to pay its singles champions equally.

This year, the U.S. Open winners will each receive $3 million, with total player compensation rising to $65 million. Tennis players accounted for seven of the 10 highest-paid women's athletes in 2022, according to Forbes.

Still, not all tournaments pay the same prize money. Novak Djokovic and Gauff both won titles at the Western & Southern Open in Cincinnati a week ago, but while Djokovic took home $1,019,335, Gauff earned $454,500.

“Even today, there are far too many tournaments out there that still need to give equal pay to women. ... Let us remember all of this is far bigger than a champion’s paycheck,” Michelle Obama said Monday. “This is about how women are seen and valued in this world.”

After the match, Gauff got to spend some time with the Obamas.

“I've met Mrs. Obama before,” she said. “They told me it was just her initially. Then Mr. Obama was there in the room, too. I was like, Oh, my God. I haven't soaked it in because I literally just walked in here. I think I'm going to never forget that moment for the rest of my life. Yeah, I went from being really upset after a win to, like, being really happy. So I'm glad I got to meet them. They gave me some good advice, too.

She added: “They just told me how I handle myself in these situations. She said it's good to speak up for myself. I think she was happy that I spoke up for myself today.”