33-year-old Donald Williams is a fighter out of the Minnesota Martial Arts Academy in Brooklyn Center ... and has a 6-6 record, according to Tapology.

Williams appeared on the stand in a Minnesota courtroom to testify about what he saw on the day Floyd died ... and his explanation was almost identical to the analysis of UFC superstar Jon Jones, who spoke out in the days following the incident.

"One, the neck was diagonal across the throat, which on a blood choke you attack the side of the neck," Williams said.

"To get the choke tighter, you hit different shimmies, which I felt the officer on top was shimmying to actually get the final choke in while he was on top to get the kill choke."

"Because a side choke or a blood choke can ultimately turn into death, which is what we've seen here."

In fact, Williams is shown in body cam footage of Tou Thao -- one of the other officers involved in the incident -- where he begs them to check Floyd's pulse ... and says point-blank Chauvin is using a "blood choke."

As we previously reported, Jones described the move in detail ... saying, "I wouldn't wish the way George Floyd was murdered on my worst enemy. That officer applied just enough pressure to keep him alive for almost six minutes in that chokehold."

"In all my years of fighting, I can honestly say I’ve never experienced anything close to that level of torture."