Usher's a Ping-Pong Master on 6th Ave

Usher's just the latest celeb to piss off drivers on NYC's 6th Ave ... only this time it was for the sake of ping-pong.

The R&B superstar obviously needs to work on his serve -- as you can see -- after getting into a quick match with jeweler to the stars Mr. Flawless ... who's made it a habit to block off the busy one-way street for photo shoots right outside his shop.

Usher proved to be a good sport though -- and apparently this publicity stunt hasn't gotten anybody busted ... at least not yet.

Check out the vid for yourself ... something tells us it won't be the last time you see Mr. Flawless getting honked at on 6th Ave.