Equal Pay Day is the symbolic day - April 2 in the United States - dedicated to raising awareness of the gender pay gap. 

It is a familiar theme for women's tennis and a cause embraced by the WTA since its foundation.

In the first part of a new video series,  'Evolution of Prize Money', presented by Cambridge Global Payments, WTA Legends look back on the early days and how the battle began.

Recalling the prize money gap at the 1968 Wimbledon Championships, Billie Jean King says: "In 1968 Rod Laver won £2,000 and I won £750, and I went, 'Oh no, we're not equal. Again."

It was a cause that united the top women's players of the time whose persistance pioneered progress from which today's players now reap the dividends.

Watch the video below for more insight into this remarkable story.