Usher, Jermaine Dupri, Tamia, Grant Hill Hang with Anita Baker Backstage

Anita Baker's Valentine's Day show in Atlanta is earning rave reviews, especially because it was capped off with a cameo surprise from Usher mid-show!!!

Backstage looked like a scene from an award show as Jermaine Dupri assembled the troops backstage for photos with the woman of the night. Tamia and Grant Hill, Chris Webber and John Gray all said cheese with Anita inside State Farm Arena ... where Usher surprised the audience and the headliner.

The "Confessions" singer brought out a huge bouquet of roses for Anita ... only to be surprised himself when she passed him the mic, putting him on the spot!!!

Usher didn't disappoint the ATLiens and belted out a few notes from Anita's classic "Same Ole Love (365 Days A Year)," and eventually worked up a sweat himself.

AB is currently on the road with Babyface .. her first nationwide tour in nearly 30 years, which doubles as a celebration for the 40th anniversary of her debut solo album, “The Songstress.”