The singer was doing yet another show at his Vegas residency Friday -- where none other than Saweetie showed up to see the man in action. Welp, like he's done with other stars in the recent past ... Usher honed in on her and started belting out slow jams right to her face.

Not the song, mind you -- he was actually crooning his hit, "There Goes My Baby," and was seeking a little assist from Saweetie ... who did help him, albeit somewhat off-key. 😅

It proved to be a sweet and funny moment, no doubt ... and Saweetie certainly seemed to be in on the fun. Of course, what everyone almost immediately thought of when seeing this was the whole Keke Palmer debacle -- where her man lost his s*** over her doing the same.

You'll recall ... Darius Jackson -- with whom Keke shares a child -- publicly aired out his grievances about Keke getting special a little treatment from Usher this past week.

His main gripe, it seems, was more so about her outfit ... which he apparently thought was a little too revealing, and which he made known by reminding her that she was a mother now.

The dude got dragged by the Internet at large, and he has since deleted his social accounts. Keke, meanwhile, also seems to have been down to stick it to her baby daddy -- doubling down in her wardrobe and posting even more sexy shots of herself in the sheer dress.

And with all that in mind, everyone was looking for a reaction from YG in this new Saweetie instance ... perhaps expecting him to be bent out of shape, but not finding a damn thing.

YG -- who Saweetie has recently been romantically connected to, and who was with the guy as recently as last week -- hasn't said anything about Usher singing to his girl ... and rightly so. There's nothing to complain about, really, and it would appear YG recognizes that.