Justin Bieber -- Works His Instrument ... Bags Playboy Chick (VIDEO)

Justin Bieber showed off his fancy finger work in the middle of a bar to serenade super hot Playboy model, Xenia Deli.

Justin and Xenia weren't afraid of showing some PDA Friday night, snuggling on a piano bench as he played for her in front of the crowd at an L.A. bar ... and then hit the clubs afterward.

Xenia is in Bieber's upcoming music video for his new single -- "What Do You Mean?" and we're told he's really into her ... introducing her to all his friends and even posting a pic of them playing footsie.

The Biebs is really stepping up his game -- not only is Xenia four years older than him, she also has SI Swimsuit and Victoria's Secret experience ... in addition to Playboy.