Lady Gaga has praised Noah Cyrus for cheering her up while she's ill by covering her track Million Reasons at a concert on Tuesday (19Sep17).

Miley Cyrus' younger sister kicked off Katy Perry's Witness tour in Montreal, Canada on Tuesday, and opted to cover Gaga's 2016 ballad as part of her support slot set.

Footage of her performance circulated online and caught the attention of Gaga, who is taking time off to work with doctors to tackle her chronic pain condition Fibromyalgia.

She then shared the footage on Twitter and thanked Noah for lifting her spirits while she is ill.

"This really cheered me up. Keep killin it lil' music angel" she wrote. "Really warmed my heart to hear you sing my song," she wrote alongside a heart emoji.

Gaga was due to start the European leg of her Joanne World Tour in Spain on Thursday, but she postponed it earlier this week to work on her health.

"I'm a fighter. I use the word suffer not only because trauma and chronic pain have changed my life, but because they are keeping me from living a normal life," she confessed in an emotional Instagram post. "They are also keeping me from what I love the most in the world: performing for my fans.

"I am looking forward to touring again soon, but I have to be with my doctors right now so I can be strong and perform for you all for the next 60 years or more."

Noah is scheduled to serve as support for Katy until 1 November (17), after which she will be replaced by Purity Ring and then Carly Rae Jepsen.

Her father Billy Ray Cyrus praised her on Twitter at the start of the tour, writing, "Congratulations @noahcyrus!!!!! Hot Damn ! I'm sooooooo proud of you! Amazing! I wanna be you when I grow up!!!"