The tennis icon stopped to talk with ET while walking the red carpet at the premiere of the upcoming HBO documentary about her pregnancy and return to the tennis court, Being Serena, and the new mom shared some kind words for her soon-to-be-royal gal pal.
"I'm obviously super happy for her," Williams gushed. "She's such a great girl and she is incredibly nice and I couldn't be happier for her."
Williams and Markle have been friends since at least January 2015, when the Suits star tweeted a photo of them hugging after Williams' win at the US Open.
In her now-deleted blog, The Tig, Markle once gushed about being friends with the tennis legend, writing, "She quickly became a confidante I would text when I was traveling, the friend I would rally around for her tennis matches, and the down to earth chick I was able to grab lunch with just a couple weeks ago in Toronto."
On May 19, Markle and Prince Harry will be exchanging vowsat St George's Chapel in Windsor Castle, just six months after Williams tied the knot with her husband, Reddit co-founder Alexis Ohanian, at a star-studded ceremony in New Orleans. Their wedding came two months after the couple welcomed their first child, daughter Alexis Olympia.
Ohanian was on hand at the Being Serena premiere to show his support for his wife, and Williams had nothing but love for her considerate husband.
"He is super supportive and has always here for me," Williams shared. "You know, all of my friends are, and it means a lot to me."
In HBO's upcoming biographical documentary, Williams says fans will get a "raw and open" inside look at her life, in "a way you've never seen me before" -- including her pregnancy journey and motherhood.
The tennis icon has been a mom for over seven months now, and she says the "most rewarding" part of the experience is "just seeing her smile every day."
"She doesn't have any teeth, and she's seven months, so it's like I get to see her toothless smile and it makes me so happy," she shared of her little girl, who already has her own adorable Instagram account, with a ton of super cute baby photos.
Earlier this year, Williams made her return to the court after more than a year since competing, and she admitted that the preparation -- both physical and mental -- and competition has been grueling.
"Sometimes, when I'm out there, it's incredibly hard. It's incredibly difficult and there's moments where I'm like, 'I don't know if this is ever gonna happen,'" she said. "But it does and I keep going and I keep trying to make it."