The 29-year-old was due to face Australian Samantha Stosur in the second round today (Wednesday), an opponent Williams beat in the Toronto Masters final on Sunday.
"It's just a bit of an aggravation because I think I've been pounding it a lot and playing a lot," she said.
The 13-time major winner also hinted that she doesn't want to worsen the injury ahead of the US Open, which begins August 29.
"I don't think this is a good time for me to take a big chance. I just don't think that would be smart."
Williams returned to tennis in June after 11 months away from the sport with health issues.