Overzealous Fan Tried Fighting Mike Tyson After Roy Jones Jr. Bout

Mike Tyson almost got into a second fight after stepping into the ring with Roy Jones Jr. -- because some wacko swung on him trying to prove something ... TMZ Sports has learned.

Sources familiar with the situation tell us ... Mike was leaving Staples Center Saturday night after his bout with Roy -- which was called as a draw -- and on the way out, he came across a small group of fans looking for autographs.

When he walked over to do some signing, we're told a man in the crowd yelled out to Mike ... asking why he wouldn't let the guy fight him. Our sources say Mike jokingly responded, joking with the dude that it probably wouldn't end well for him.

Next thing you know, we're told the dude swung at Mike ... and then proceeded to reach into his waistband, as if trying to pull out a weapon. Our sources say Mike's team jumped in right away before things got ugly.

We're told the guy took off -- and our sources also say some folks on Mike's team recognized him from a previous signing, where they say he also tried starting trouble.

Law enforcement sources tell us cops got a call about a possible battery involving Mike Tyson around 11 PM PT (well after Mike tussled with Roy) ... but when they responded to the scene, all parties had left and no report was taken.