Tiffany Haddish Grinds on Usher at Vegas Residency Show

Tiffany Haddish got up close and personal with Usher this weekend -- receiving the serenade treatment that also teetered into touchy-feely territory.

The actress/comedian was in attendance for Usher's Vegas residency show Friday night, where she had a great seat toward the front ... but which got a whole lot better when he and his team actually invited her up onstage.

Seems like he knew she was in the crowd and wanted to giver something special ... she got it and then some. Check out the video -- TH looks like a kid in a candy store up there.

As you can see ... Usher did a little dance with Tiff, grinding up on her backside and her totally leaning into it. He's also singing to her -- and eventually, has her sit down on a table on stage as they begin to act out a skit. Tiffany's a pro, so she rolls with it all to a T.

Sure looks like she had a good time -- it's also just crazy to see that Tiffany seems pretty star-struck up there. Just speaks to the fame and allure that Usher holds to this day.

Another thing this confirms ... Usher's residency show is still the hottest thing in Sin City.