UFC fighter, Alexander Hernandez, told the media he wanted to "party his d*** off" after nearly flatlining Mike Breeden ... and while he says the D is "intact", the 29-year-old fighter did run up a MASSIVE tab in a Sin City club.

TMZ Sports talked to Hernandez following his 1st round knockout at UFC Fight Night ... and asked him about the nights festivities.

"D*** is intact, but the partying was out of control, dude."

"I wanted to just go to old town Vegas and hang out with the locals, and just do simple pleasantries, and just have a good time out there, walk past some shady guy, hand me some s*** in the bathroom, and I'll be like, 'Okay, whatever, in Vegas.'"

But, the Fremont wasn't meant to be, and Alex says he got "coaxed into some goddamn Tiesto club party ..."

Hernandez says they ended up getting a table ... and they dropped an insane amount of cash.

"I got a text message about a $23,000 tab. And, I'm like, 'the AT&T user you're trying to reach is no longer in service.' You're gonna have to speak to somebody else about that."