In a stunning snub to the first Australian woman to top the rankings in 43 years, Williams said she wasn't aware Barty had become No.1 until being told at her pre-Wimbledon press conference on Saturday.
Williams, though, did say she was happy for Barty and was gushing in her appraisal of the Queenslander's game.
"Wow, that's great," the 23-times grand slam champion said.
"I think Ash, I don't know anyone that has anything negative to say about her. She's like the sweetest, cutest girl on tour. She's so nice.
"She has the most beautiful game, such classic shots. I mean, she does everything right. Her technique is, like, flawless.
"Obviously I'm happy for her. Yeah, it's good. It's good for her."
The American superstar endorsed Barty as a genuine chance of becoming Australia's first Wimbledon singles champion since Lleyton Hewitt in 2002.
"She has a great game. I think, like, she's really even-tempered," Williams said.
"She's just really chill. I feel like she's just so relaxed in a way. She's had a solid year, as well. It's not just the French Open. I believe she won Miami. She's had a really good year.
"Yeah, I think anything is possible."
Ashleigh Barty. Source: Getty Images Europe
Barty acknowledged it was a "new feeling" entering the All England Club as the top seed.
"Something that I've never experienced before," she said.
"It hasn't really changed much, to be honest. We're still trying to go about all of our business, all of our preparations the same way.
"We know what we've been doing has been working. For us, it's about trying to keep that normality as much as possible.
"There's more attention, there's more of that outside noise. But from what we're trying to do on the court, it hasn't changed much. We'll just keep trying to grow and be better every single day."
Barty and Williams could clash in the quarter-finals, but Williams isn't looking that far ahead, let alone the prospect of matching Margaret Court's all-time record of 24 singles majors.
"I can't go into grand slams thinking about records," said the seven-times champion and last year's runner-up to Angelique Kerber.
"I just have to go in a grand slam and think about just the first match.
"If I get that, I think about the next match. If I'm in the final, try to win one for once. That's kind of what I think about."
