Cooper and Gaga are both nominated in the main acting categories at the Academy Awards, along with Shallow, which is up for best original song.
Lady Gaga's voice is also expected to help make her Vegas residency wildly successful.
A Star Is Born tells the tragic love story of ME, played by Cooper, who helps Gaga's struggling artist Ally.
If you attend a Lady Gaga performance, you should no doubt be ready for uninmagineable surprises. Bradley and Gaga pointed at each other, indicating their appreciation for one another's talent.
The singer then took a seat at the piano and began playing as Cooper stood up and leaned over the instrument, joining her in singing the chorus.
In video clips shared on social media from concert-goers, Gaga sits at the piano while Cooper is on a stool as they sing the Oscar-nominated track together.
Either way, check out footage from Gaga and Cooper's performance below, courtesy of Garret Gagnon and Gaga Daily. Much to the crowd's delight, Bradley threw in a little air guitar as his co-star finished up the song with its famous dramatic ending.