The Duchess of Sussex and Prince Harry made a series of astonishing revelations about their life in the two-hour broadcast, which was broadcast on Sunday night.
Writing on Twitter, the tennis star praised her “selfless friend” who “teaches me every day what it means to be truly noble”.
She added: “Her words illustrate the pain and cruelty she’s experienced.
“I know first hand the sexism and racism institutions and the media use to vilify women and people of colour to minimise us, to break us down and demonise us.
“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.
“I want Meghan’s daughter, my daughter and your daughter to live in a society that is driven by respect.
“Keep in your memory the fruitage of the spirit is love, joy, peace, long-suffering, kindness, goodness, faith, mildness, self-control. Against such things there is no law.”
Daniel Martin, who did Meghan’s make-up on her wedding day, shared a picture of her with Prince Harry on Instagram, alongside a poem from Maya Angelou.
It read: “You may write me down in history With your bitter, twisted lies, You may trod me in the very dirt. But still, like dust, I’ll rise.”
Celebrities and activists have also shown support for the couple, including the niece of Vice President Kamala Harris, Meena Harris.
Prior to the interview airing, she tweeted: “Happy Meghan Markle appreciation day.”
After it had finished she said: “Her Majesty The Queen Oprah.”
During the two-hour interview, Meghan told Oprah she had contemplated taking her own life at the height of her crisis with the monarchy.
She also revealed there were “several conversations” about her son Archie’s skin tone.
Poet Amanda Gorman, who read her piece The Hill We Climb at the inauguration of US President Joe Biden, said Meghan was “the Crown’s greatest opportunity for change, regeneration and reconciliation in a new era”.
“They didn’t just maltreat her light – they missed out on it,” she added.
Bernice King, the youngest child of the late civil rights activist Martin Luther King, tweeted: “Royalty is not a shield from the devastation and despair of racism. I’m grateful that #MeghanMarkle is still here.”
CBS Presents Oprah with Meghan and Harry airs on ITV at 9pm tonight and on ITV Hub.