The former X Factor winner got glammed up and took to the stage in NYC on Monday night (10 June) at the Apollo Spring Gala, which was held in honour of funk-soul singer, Chaka Khan.
Alex, 24, wore a racy figure hugging navy blue dress for the red carpet that was daringly short at the front and featured fringed detailing at the back to give the illusion of a floor length gown.
She also went all out with her hair style by sporting huge curls and completed her look with a pair of silver heels.
The London raised songstress - who was the only British artist on the line-up - then got changed into another sparkly blue number and performed in front of hundreds of guests and fellow celebs including Sarah Jessica Parker, Patti LaBelle and Mary J. Blige.
She was even joined on stage at one point by famed Dreamgirls star, Jennifer Holliday, and chart-topping Canadian R&B singer, Deborah Cox, for a show-stopping performance that no doubt did Chaka Khan and Alex's fans in the audience proud.
Sharing a stage with such huge icons in the music industry proved to be a little surreal for Alex, who admitted afterwards that she couldn't believe the night had happened.
She tweeted: "Still can't believe my luck... Can't believe I'm apart of this line-up. Lil old me from london...wowza."
"This is soooo overwhelming !! I'm unsure if this is real... I'm so thankful for this moment today..."
Earlier this year, the singer parted ways with her record label, RCA, but has since been working hard in the studio on new music.
She is now staying in New York for two months to record her upcoming third album and recently signed a new management deal with Kevin Liles and KWL, the ex Def Jam president who currently manages artists such as Nelly, Trey Songz and Estelle.
What do you think of Alexandra's stage outfits? Let us know below!
Copy Hilary Duff's glam ombre glitter manicure
Cheryl Cole admits her mum would "go crazy" if she got Botox!