David Foster & Christie Brinkley Sing at TMZ Photog's Birthday Party

David Foster is a man of his word ... he not only showed up at a birthday party for one of our camera guys, but he brought Christie Brinkley -- and they both sang!

The legendary songwriter/producer walked into the Stinking Rose, grabbed a seat at the piano and started playing a string of hits ... including "I Will Always Love You" and "Got to Be Real." In honor of Christie, he even launched, briefly, into Billy Joel's "Uptown Girl."

There's been buzz about David and Christie dating for a few months. They didn't confirm anything, but c'mon ... he brought her to a TMZ photog's party. That's gotta seal the deal.

Foster also brought his singer pal, Sheléa Frazier ... that's her doing "Got to Be Real."

Our guy, Kris, first mentioned his party to David on a lark at Craig's a couple weeks ago. We'll say it ... we didn't believe he'd actually do it, but here's proof David's not your typical entertainment icon.