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

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.' "