The US beat Japan to win the 2015 Women's World Cup

Serena Williams has voiced her support for the US women's football team in their legal battle for equal pay.

All 28 members of the national squad have joined a class action lawsuit against the US Soccer Federation.

It says they should not be paid less than members of the US men's team.

"I do know that the pay discrepancy is ludicrous," said Williams, a 23-time Grand Slam tennis champion. "So what they are doing right now is hopefully for the future of women's soccer."

Williams highlighted the campaign for equality in tennis in the 1970s by the likes of multiple Grand Slam winner Billie Jean King. "I'm playing because someone else stood up," she said.

She told reporters: "We have had some incredible pioneers in our sport that stood up in the '70s and said... 'We're going to get paid what the men get paid'.

"I think at some point, in every sport, you have to have those pioneers, and maybe it's the time for soccer."

The US players' union - the United States National Soccer Team Players Association - said it supported the women's team and "the efforts to achieve equal pay". The USSF is yet to comment on the legal action.

The US finished second to England in the SheBelieves Cup and will be looking to retain the World Cup, which takes place between 7 June and 7 July in France.