Talk about a knock out!

On Sunday's Keeping Up With the Kardashians, Kim Kardashian steps into the ring for a charity boxing match -- with harsh results.

She ends up taking several punches to the face.

"I've never been hit so hard," Kim, 29, says in the clip (above).

Despite her defeat, Kim's sister Khloe -- both of whom grace the new cover of Us Weekly (on newsstands now) -- was proud of her sib.

"I'm really impressed," she says. "Kim looks aggressive out there -- she's defending herself!"

The full episode of Keeping Up With the Kardashians airs Sunday at 10 p.m./ET on E!.