The veteran rocker delighted fans on Wednesday night (20Jul16), during his monthly residency at the fabled Madison Square Garden venue, by introducing Tony to join him for a duet on Joel's classic New York State of Mind.

However, the surprises didn't stop there, as Billy then led the 18,000-strong crowd in a rendition of Happy Birthday, in honour of his fellow Big Apple icon's upcoming age milestone.

Joel also presented the birthday boy with a black-and-white cake, bearing his image, to mark the occasion.

The celebration was a little premature, as Tony doesn't turn 90 until 3 August (16).

According to Newsday.com, the crooner is expected to spend his big day with friends and family with a private party at the city's famed Rainbow Room restaurant, although reports suggest plans are being made for Tony to celebrate with fans too later this year (16).