Billy Joel sang 'Happy Birthday' to Tony Bennett at a gig on Wednesday (20.07.16) night.

The 'Uptown Girl' singer was joined at his monthly gig at New York's Madison Square Garden by the veteran crooner - who turns 90 on August 3- to sing 'New York State of Mind', and he ensured he marked his guest's upcoming milestone by instigating a crowd singalong before he left the stage.

Tony applauded the fans and blew kisses as they sang, and Billy thanked him as he walked away.

Backstage, Billy presented his pal with a black-and-white birthday cake with the 'Lady is a Tramp' singer's face and a microphone on it.

Tony tweeted afterwards: ''Thanks for the early birthday cake @billyjoel! It was such a thrill to perform with you last night.''

The legendary entertainer is expected to spend his actual birthday with friends and family at a private celebration at the Rainbow Room, but plans are reportedly underway for a public event to mark the occasion later this year.

Meanwhile, Tony has spoken of how the famous 'Whole Earth' photograph - which is also known as the Blue Marble - that was taken in 1972 by the crew of Apollo 17 from a distance of 28,000 miles changed his life as it made him realise ''every problem should have a solution'' because we're all united in one planet.

He told Vanity Fair: ''All my life I've been a humanist. But what really put this into focus for me was the day, 44 years ago, when I saw this photograph for the first time: our planet Earth, taken from outer space. I remember being so captivated, thinking about what it must have been like to frame the entire world between two fingers. What a perspective. All the tribulations, the wars, the prejudice--and everything that divides us--simply melt away when you realise that we're all together on one planet and that every problem should have a solution.

''As Ella Fitzgerald used to say to me, 'Tony, we are all here.' ''