Sir Elton has recorded a duet with Spears, called Hold Me Closer, which will be released on Friday.
However, he couldn't resist sharing the track with the patrons of a restaurant in Cannes on Tuesday night.
After approaching the restaurant's DJ, he sang along with the track, streaming the reaction on his Instagram page.
Wearing a colourful shirt and matching shorts, Sir Elton, who had hip surgery last year, remained seated for the performance.
Hold Me Closer is a mash-up of Sir Elton's hits The One and Tiny Dancer, with Spears singing lyrics from both songs over a bouncy club beat.
In style and sound, it's similar to Sir Elton's duet with Dua Lipa, Cold Heart, which became a global number one hit last year.
That track combined elements several Sir Elton classics, including Rocket Man, Sacrifice and Kiss The Bride, and was nominated for single of the year at the 2022 Brit Awards.
The veteran star also previewed that song with an impromptu karaoke performance at the La Guerite beach restaurant in Cannes.
Hold Me Closer will be Spears' first new music since her 2016 album, Glory. It was reportedly recorded last month in Los Angeles.
The duet was "Elton's idea and Britney is a huge fan", said US tabloid the NY Post, citing an unnamed "industry insider".
Sir Elton previously revealed the cover art for the single, which features side-by-side photos of the two stars as children - Spears in a pink tutu and Sir Elton behind a piano in his school uniform.
Last November, Spears was freed from the controversial conservatorship that controlled almost every aspect of her life for 13 years.
It meant the star could once again take control of her finances, career and personal life, and would no longer have to pay a team of professionals and lawyers to oversee her affairs.
Although fans have been clamouring for her to return to music, the star has been reluctant to take that step.
In a series of Instagram posts last year, she said she was "staying clear" of the music industry.
"People have no idea the awful things that were done to me personally," she wrote. "And after what I've been through, I'm scared of people and the business!"