The retired tennis star said she first discovered the growth, which was found to be non-cancerous, in May and recently had it removed on the advice of doctors.

Doctors told Williams it was a "brachial cyst" and didn't need to be removed. However, the cyst kept growing, so Williams underwent three more tests and a biopsy. The cyst is benign, but she still decided to have it removed so it wouldn't get infected or leak.

A branchial cyst, a.k.a. a branchial cleft cyst, is a small, fluid-filled sac that an look like a lump under the skin on the side of your neck, according to the Cleveland Clinic. They can appear at any age, but are usually found in kids.