Usher wrapped up his Vegas residency with a ton of appreciation -- which you could see as the dude fell to his knees and took in all the love in a truly emotional moment.

The singer went out with a bang Saturday in Sin City, where his 'My Way' shows finally came to an end after an epic 100-concert run -- most of which have been very eventful ... for a variety of reasons. This weekend though ... all the focus was on Mr. Raymond.

While he was singing "Without You" ... he buckled and started to break down onstage -- this while his backup dancers and other stage crew members rallied around him as the crowd cheered. Indeed, it was quite something ... and you could tell he was overcome by it all.

Of course, an Usher show wouldn't ring true if he wasn't out there serenading the crowd -- and he did just that ... crooning to Jennifer Hudson (who was with Common) the night prior.

Other stars in attendance on Sat included Mary J. Blige, H.E.R., Floyd Mayweather and others ... and it looks like they all enjoyed the man himself for one last hurrah before he hangs it up.

Of course, while this might be the end of Usher in Vegas ... he'll undoubtedly continue performing -- as this lengthy stint proved there's a real appetite for all his hits ... not to mention his sweet dance moves and stellar showmanship.