Serena Williams said she understood the "pain and cruelty" suffered by Meghan, the wife of Prince Harry, after she accused Britain's royal family of racism.
Williams, who has won 23 Grand Slam titles, said her "friend" Meghan had taught her "what it means to be truly noble".
"I know firsthand the sexism and racism institutions and the media use to vilify women and people of colour to minimise us, to break us down and demonize us," Williams tweeted.
Tokyo 2020 men
Furious Medvedev smashes racket after Olympic quarter-final defeat to Busta
"We must recognise our obligation to decry malicious, unfounded gossip and tabloid journalism.
The mental health consequences of systemic oppression and victimisation are devastating, isolating, and all too often lethal.
Meghan and Harry said in the interview their second child, who is due this summer, would be a girl.
"I want Meghan's daughter, my daughter, and your daughter to live in a society that is driven by respect," Williams added.
Tokyo 2020
'That was a performance!' - Djokovic bagels Nishikori to storm into semi-finals
Tokyo 2020
'I had all the answers' - Djokovic 'very happy' after thrashing Nishikori