“Basketball Wives” star Jennifer Williams‘ feud with Evelyn Lozada‘s assistant ain’t over — TMZ has learned, the assistant is now taking their beef to the police … accusing Jennifer of stealing her house keys.

The assistant – Nia Crooks (right) — filed a theft report against Jennifer Monday with the Halladale Police Department, claiming JW is refusing to return the house keys Nia loaned her two months ago … back when they were still friends.

TMZ broke the story – Jennifer filed a battery report against Nia last month, claiming the assistant got violent with her at a “Basketball Wives” shoot at some Florida racetrack … and we’re told, tensions have been running high ever since.

According to sources, Jennifer has been making thinly-veiled threats to Nia since the racetrack scuffle — things like “watch your back” — and Nia doesn’t feel comfortable with Jennifer having a key to her house.

We’re told Nia has repeatedly asked Jennifer to return the keys to no avail — so she’s turning to law enforcement for help, completely baffled by Jennifer’s behavior.

Calls to Jennifer have not been returned.