Leistner and his Hamburg squad had just lost to Dynamo Dresden in a 4-1 shellacking in the German Cup when he heard someone spewing insults from the crowd.
The 30-year-old stopped a live TV interview and jumped into the stands, walked right up to the man he believed to be the trash-talker ... grabbed the dude by the shirt with both hands and shoved him to the ground.
Fans and security broke up the altercation -- and Leistner went right back to his interview and picked up like nothing happened.
The soccer star later went to social media and explained why he blew up ...
"After the match in my hometown, I was verbally abused from the stands," Leistner said.
"I can usually cope with it. But this was extremely and massively below the belt, against my family, my wife and my daughter."
"That's when I lost it, even more so because it was already an extremely emotional match for me."
He continued, "Regardless, this should have never happened to me. I am a father who wants to serve as a role-model. I apologize in all forms and can only promise that -- no matter what insults are thrown my way -- this will never happen again."
As for the COVID component -- roughly 10,000 fans were allowed inside the stadium to watch the game ... but were all required too wear masks and engage in social distancing.
The platers were tested before being cleared to play -- but the fans were not tested.
Now, after this skirmish ... you would imagine Leistner will be tested again soon and hopefully, he's okay.