Sources close to the production confirm eyewitness accounts that AK rehearsed with Usher inside Allegiant Stadium on Thursday. Now that we know she'll be in the mix -- the first confirmed guest of his BTW -- all signs pointing to them doing one big song together.

Of course, we're talking about their 2004 mega duet "My Boo" -- which will undoubtedly be at least one of the tracks she performs.

The song topped the Billboard Hot 100 for 6 weeks when it came out -- and became the soundtrack to high school slow dances worldwide. It'll be interesting to see where the slow jam is included in the track list ... but he'll have plenty of time to squeeze it in.

Remember ... Usher negotiated to have a 15-minute set as opposed to the originally allotted 13 minutes ... 'cause it's the only way he knows how to rock, it seems. Considering how many famous collaborators Usher has had over the years ... it makes sense he needs the extra time.

His Apple Music commercial to promote the big event also heavily featured his "Yeah!" collaborators Ludacris and Lil Jon. Most everyone assumes these two guys will also be special guests during his performance ... although, they technically haven't confirmed yet.

As you know ... Usher has also been talking to Justin Bieber as a potential guest onstage -- although we haven't locked down whether the Biebs will actually be in attendance or not.