Celine Dion Sings Backstage at Grammys Amid Stiff-Person Syndrome Battle

Here's an uplifting sign of just how good Celine Dion's feeling these days -- not only did she make a surprise appearance onstage at the Grammys, but backstage she gave an impromptu performance!

The icon showed off her famous vocal chops alongside Grammy-nominated singer-songwriter Sonyaé in a clip filmed backstage Sunday night at L.A.'s Crypto.com Arena -- proving she's still got it amid her battle with Stiff-Person Syndrome, which makes performing difficult due to painful muscle stiffness and spasms.

Celine was effortlessly smooth in her delivery while groovin' along with Sonyaé ... this marking the first time she's sung publicly in 4 years, making the moment all the more special.

Sonyaé shared the IG clip ... rightly captioning it as one of her most iconic moments from the evening, which went down a treat with her followers.

It was a big night for Celine ... who was met with a standing O while presenting the Album of the Year Award -- her industry peers were clearly thrilled to see the hitmaker out, who has mostly been staying out of the spotlight since announcing her rare neurological syndrome.

Although she looked and sounded amazing, sources with direct knowledge tell us the sad reality for Celine is that she still struggles physically many days. In December, her sister said Celine remained focused on her desire to return to the stage.

That explains the joy on Celine's face Sunday night ... as she put it before presenting the Album of the Year, "When I say I am happy to be here, I really mean it. From my heart."

Of course, that heartwarming moment turned awkward moments later when Taylor Swift bizarrely ignored her as she rushed onstage to collect her award.

However, no one dares disrespect the Queen of power ballads, not even Tay Tay herself.