Boxing Star Shakur Stevenson Fighting GF's Brother, 'No Mercy!'

How's this for AWKWARD -- undefeated boxing phenom Shakur Stevenson is fighting his GF's brother on Saturday night ... and he's vowing to bust that dude up!!!

Don't get it twisted, Shakur's opponent is no bum -- he's undefeated 23-0 Joet Gonzalez -- and they're fighting for the WBO featherweight championship!!!

So, how does 22-year-old Shakur feel about trying to knock out his girlfriend's sibling? NOT BOTHERED AT ALL!!!

"He's bothered by the situation," Shakur says. "He's the one who hates me. I don't really hate him."

We asked Shakur if he was going to take it easier on Joet because of the family connection -- and man, he straight up laughed in our faces.

"Hell no! F**k no! I'm not taking it easy at all!!!"

In fact, Shakur says ... "Saturday night I've got something that's gonna make him more mad!"

By the way, Shakur has been dominant in the ring ... and has HIGH HOPES for his pro career -- even calling out Vasyl Lomachenko, widely considered one of the best pound-for-pound fighters on Earth.

"Lomachenko. Lomachenko. Lomachenko is my dream fight."

Shakur says the guys have been in the ring together a lot ... 'cause he sparred with Vasyl back in 2017.

The dream could become reality very soon ... as in like the next 2 years, according to Shakur.

"I actually respect Lomachenko, he's one of the people that I think are one of the best in boxing. He's one of the people that I want to take their spot."