Ex-NFL running back Marcel Shipp just caught a huge break in his domestic violence case ... 'cause the charges were dropped after a key witness failed to show in court Monday ... TMZ Sports has learned.
Shipp -- a former Cardinals RB from 2001 to 2007 -- was facing over a year in prison after he allegedly got physical with a woman on a sidewalk in downtown Scottsdale in February.
40-year-old Shipp was hit with 3 misdemeanor charges -- including assault and disorderly conduct.
A hearing in the case was scheduled for Monday morning -- but the key witness was a no-show, prosecutors say.