The Piano Man, BILLY JOEL and ex- wife CHRISTIE BRINKLEY’s daughter ALEXA RAY JOEL collapses on stage during gig at the Café Carlyle in Manhattan.

Joel, 28, told audience members she was feeling ill, having thrown out her neck and back before she collapsed in front of the stunned audience Saturday night.

Alexa was rushed to New York Presbyterian Hospital Emergency Room, where she was diagnosed with Vasovagal Syncope – the cause of fainting, the NY Post reported.

Vasovagal Syncope trigger causes a sudden drop in heart rate and blood pressure, which leads to reduced blood flow to the brain — quickly resulting in a brief loss of consciousness.

The fairly common condition is considered harmless, requiring no treatment.

In a statement Alexa said, “I wanted to thank everyone for coming out to support me. I was excited and determined to fulfill my final performance and I really wanted to end my run with a bang, but this was not what I had in mind and hope I have the opportunity to make it up to the wonderful audience very soon.  Thanks for all the concern.”

Alexa was recently under scrutiny for debuting a hot new look that many were saying was the result of drastic plastic but Joel had pooh-poohed THAT by citing a “wonderbra” and new makeup.