The two locked eyes during the performance of Cooper's verse, with Gaga eventually looking at the crowd with a smile as if to say: "Can you believe he just did that?"
Cooper rocked out behind her, playing an air guitar as Gaga sang the emotional ballad.
Lady Gaga's voice is also expected to help make her Vegas residency wildly successful. Despite a rocky start in which the actor joked about having to adjust his in-ear monitor - "Jackson never used these", he quipped - the rest of the emotional number went smoothly. He looked into Gaga's eyes as he sang the lyrics that she wrote for the film.
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Near the end of the duet Cooper reportedly sat next to Gaga and wrapped his arm around her waist.
Though he added, "I can't hear anything", Cooper seemed unfazed as he launched into a rendition of the song, which is nominated for an Oscar.
The song, which was nominated for an Academy Award earlier this week, is the closing number of the "Enigma" show, the residency that kicked off in December past year.