Fans who attended Lady Gaga's Las Vegas show Saturday night were treated to a star-studded surprise when Bradley Cooper appeared onstage — and the pair sang the duet, "Shallow," from their hit movie "A Star is Born." 

Cooper, who starred in and directed the 2018 film, was attending a performance of Lady Gaga's "Enigma," one of two shows in her Las Vegas residency at Park MGM resort, when she apparently called him out of the crowd.

Video: https://youtu.be/zfeHACNRs20

"So a really good friend of mine ... I don't know if you know him, but his name is Bradley. He made that movie, 'A Star Is Born,'" Lady Gaga said in a video posted to YouTube. "So, I'm gonna be a little chance-y .... Yo Bradley, you wanna come up and do this one?"

The audience went wild as Cooper reluctantly joined Lady Gaga next to her piano and sang the Golden Globe-winning duet.

The stars, who play Ally and Jackson Maine in the flick, had never belted out the song together in front of a live audience outside of the movie, reports Rolling Stone. However, it is likely the pair will be asked to perform at the Academy Awards next month.

"A Star is Born" is up for eight Oscars, including best picture, original song and best actor and actress nods for both Cooper and Lady Gaga.

BEST PICTURE
"Black Panther"
"BlacKkKlansman"
"Bohemian Rhapsody"
"The Favourite"
"Green Book"
"Roma"
"A Star is Born"
"Vice"

ACTOR IN A LEADING ROLE
Christian Bale (“Vice”)
Bradley Cooper (“A Star Is Born”)
Willem Defoe (“At Eternity’s Gate”)
Viggo Mortensen (“Green Book”)

ACTRESS IN A LEADING ROLE
Yalitza Aparicio (“Roma”)
Glenn Close (“The Wife”)
Olivia Colman (“The Favourite”)
Lady Gaga (“A Star Is Born”)
Melissa McCarthy (“Can You Ever Forgive Me?”) 

ACTOR IN A SUPPORTING ROLE
Mahershala Ali (“Green Book”)
Richard E. Grant (“Can You Ever Forgive Me?”)
Sam Elliott (“A Star Is Born”)
Sam Rockwell (“Vice”)
Adam Driver (“BlacKkKlansman”)

ACTRESS IN A SUPPORTING ROLE
Regina King (“If Beale Street Could Talk”)
Amy Adams (“Vice”)
Marina De Tavira (“Roma”)
Rachel Weisz (“The Favourite”)
Emma Stone (“The Favourite”) 

DIRECTING
Spike Lee (“BlacKkKlansman”)
Pawel Pawlikowski (“Cold War”)
Yorgos Lanthimos (“The Favourite”)
Alfonso Cuaron (“Roma”)
Adam McKay (“Vice”) 

WRITING (ADAPTED SCREENPLAY)
"The Ballad of Buster Scruggs"
"BlacKkKlansman"
"Can You Ever Forgive Me"
"If Beale Street Could Talk"
"A Star is Born"

WRITING (ORIGINAL SCREENPLAY)
"The Favourite"
"First Reformed"
"Green Book"
"Roma"
"Vice"

CINEMATOGRAPHY
"Cold War"
"The Favourite"
"Never Look Away"
"Roma"
"A Star is Born"

ANIMATED FEATURE FILM
"Incredibles 2"
"Isle of Dogs"
"Mirai"
"Ralph Breaks the Internet"
"Spider-Man: Into the Spiderverse"

DOCUMENTARY (FEATURE)
"Free Solo"
"Hale County This Morning, This Evening"
"Minding the Gap"
"Of Fathers and Sons"
"RBG"

DOCUMENTARY (SHORT SUBJECT)
"Black Sheep"
"End Game"
"Lifeboat"
"A Night at the Garden"
"Period. End of Sentence."

FOREIGN LANGUAGE FILM
“Capernaum” (Lebanon)
“Cold War” (Poland)
“Never Look Away” (Germany)
“Roma” (Mexico)
“Shoplifters” (Japan) 

MUSIC (ORIGINAL SCORE)
“Black Panther”
“BlacKkKlansman”
“If Beale Street Could Talk”
“Isle of Dogs”
“Mary Poppins Returns” 

MUSIC (ORIGINAL SONG)
"All the Stars"
"I'll Fight" RBG
"The Place WHere Lost THings go" Mary Poppins
"Shallow"

SHORT FILM (ANIMATED)
“Animal Behavior”
“Bao”
“Late Afternoon”
“One Small Step”
“Weekends” 

SHORT FILM (LIVE ACTION)
"Detainment"
"Fauve"
"Marguerite"
"Mother"
"Skin"

FILM EDITING
“BlacKkklansman”
“Bohemian Rhapsody”
“The Favourite”
“Green Book”
“Vice” 

SOUND EDITING
“Black Panther”
“Bohemian Rhapsody”
“First Man”
“A Quiet Place”
“Roma” 

SOUND MIXING
“Black Panther”
“A Star Is Born”
“Bohemian Rhapsody”
“Roma”
“First Man” 

PRODUCTION DESIGN
"Black Panther"
"The Favourite"
"First Man"
"Mary Poppins Returns"
"Roma"

COSTUME DESIGN
“The Ballad of Buster Scruggs”
“Black Panther”
“Mary Poppins Returns”
“The Favourite”
“Mary Queen of Scots” 

MAKEUP AND HAIRSTYLING
"Border"
"Mary Queen of Scots"
"Vice"

VISUAL EFFECTS
"Avengers: Infinity War"
"Christopher Robin"
"First Man"
"Ready Player One"
"Solo: A Star Wars Story"

The 91st Annual Academy Awards will air live from the Dolby Theatre in Hollywood on ABC Sunday, February 24th.