It all started when the reigning Wimbledon champion asked room service for dog food for her Yorkie, Chip.
"It was like, fresh greens, and boiled chicken and beef, and it was seared like tenderly, and it was just perfection," she said.
Williams was apparently enticed by the fancy meal.
"I was like ... that ... looks better than my food. ... Before you judge me, look at it," she said.
'What the heck, I'm gonna try a piece. It looks good ... Don't judge me."
After eating a spoonful, Williams regretted it.
"Let's fast forward to two hours. I just ran to the toilet like I thought I was gonna pass out. ... And now I feel really sick," she said.
Williams didn't seem worried the incident would affect her performance during Thursday's match.
"I guess tomorrow when I step on the court, I'm gonna look svelt."
A writer for LiveScience explains why you shouldn't eat pet food:
"Dog food contains many of the same ingredients as human food, like chicken, meat, and vegetables, but it may also contain animal by-products — for example, ground-up animal bones or organs like the intestines."
As for Williams, this is one of those times where the phrase "when in Rome" doesn't apply.