Venus Williams Fingers Unknown Third Driver for Causing Fatal Accident

Venus Williams has a new defense in her fatal car accident, and she's laying blame squarely on a third driver who drove away from the scene unscathed ... TMZ Sports has learned.

Venus filed new docs in the case which reference an unidentified driver in a dark-colored sedan. She says that driver, who was heading in the opposite direction, violated her right of way by cutting in front of her in the intersection ... to make a left turn.

What she's describing is clear in the surveillance video TMZ Sports first posted.

Venus says that other driver's "improper left hand turn" made her slow down, and stop to avoid hitting the sedan.

The video shows what happened next. Venus continued through the intersection, but police say by that time, the light had turned red ... hanging Venus out to dry in the intersection. Seconds later Linda Barson t-boned Venus' SUV. Barson's husband later died from his injuries in the accident.

In the docs, Venus says if she's at fault at all for the wreck ... the unidentified sedan driver should at least share the blame.