The 36-year-old says she was in awe when Middleton stepped out of the hospital and took pictures for the press just moments after giving birth to her and Prince William's third child, Louis, in April.
"She was … standing!" Williams added. "So, she’s officially No. 1."
Williams is a close friend of Meghan Markle's. She previously revealed that she is happy the former Suits actress found love in Prince Harry.
"I'm obviously super happy for her," she told ET. "She's such a great girl and she is incredibly nice and I couldn't be happier for her."