Serena Williams and other prominent American tennis players of past and present on Friday expressed support for a lawsuit filed by the US women's national football team against their federation that alleges gender discrimination in wages and conditions.
All 28 members of the United States squad were named as plaintiffs in federal court in Los Angeles on International Women's Day.
Serena said pioneers in her sport stood up for pay equality in the 1970s and that the time may be right for football to level its playing field.
"The pay discrepancy is ludicrous," the 23-time Grand Slam singles champion told reporters after her second-round victory at the BNP Paribas Open.
"It's a battle, it's a fight," she said.
"I think at some point, in every sport, you have to have those pioneers and maybe it's time for soccer," she said.
"I'm playing because someone else stood up and so what they are doing right now is hopefully for the future of women's football."
The best known of those pioneers, Bille Jean King, also welcomed the lawsuit while attending the tournament in the Southern California desert.
"What better day than International Women's Day for this lawsuit," the 75-year-old, who won 12 Grand Slam singles titles, posted on Twitter.
"Sports are a microcosm of society. What is happening with the USWNT is happening in the workplace. The time has come to give these athletes what they deserve: equality."
The work of King and her allies resulted in equal prize money for women and men at all four Grand Slams in 2007 when Wimbledon fell in line.
The issue still occasionally surfaces, however, and Indian Wells tournament chief Raymond Moore quit under a cloud in 2016 after saying women's tennis rode "on the coat-tails of the men".
Former US Open champion Sloane Stephens also sent her encouragement to the football team on Friday.
"I think there should be equal pay across the board for everyone, male or female," the world number four said.
"But I'm happy for them. I hope they win."
The thrust of Friday's lawsuit was that the women had consistently been paid less money than their male counterparts even though their performance had been superior.
The US won the 2015 women's World Cup, a title they will defend in France in June, while the men failed to qualify for the last edition of FIFA's showpiece event in Russia in 2018.