Billy Joel Throws 90th Birthday Bash for Tony Bennett

Billy Joel's celebration of Tony Bennett's 90th birthday had some serious New York flavor ... and about 18,000 people singing "Happy Birthday."

Tony hopped onstage to duet with Billy on his classic, "New York State of Mind" -- which would've been an awesome enough toast -- considering both guys are born and bred New Yorkers.

But then the entire crowd serenaded Tony and Billy even brought a cake for his pal -- he was off by a few candles though.

Some Billy fans thought he had another surprise guest ... AC/DC ... when he busted out "Highway to Hell" -- but it was just Billy's roadie, Chainsaw, doing a cover. Too bad, so sad.

Chainsaw rocks though.