Bradley Cooper joined Lady Gaga on stage during the latest show of her Las Vegas residency on Sunday night.
The 44-year-old actor - who starred alongside Gaga in the critically acclaimed movie 'A Star Is Born' - joined the chart-topping singer on stage for her show in Sin City, where they performed a rendition of the song 'Shallow', which features in the film.
Gaga, 32 - who played a young singer in 'A Star Is Born' - is currently in the midst of a residency at the MGM Park Theater, where she started her run of shows in December.
The singer has repeatedly praised Bradley's directing abilities since the movie's release in 2018.
She recalled: "We're having this extremely emotional, awful conversation with each other and - at the same time - you have to completely trust your director.
"In a moment where I'm almost un-trusting of him and angry with him and insulted by him, I'm also, in the back of my mind, in the space of comfort, in the space of love.
"What was very special about that scene, for me anyways, was the themes of alcoholism and the theme of co-dependency and addiction - that is something that has affected me in my life. To share that with him was very, very special to me. He really honoured that."
Meanwhile, Bradley recently admitted he feared his directorial debut would "fail miserably".
He said: "I was just hoping that I wouldn't fail miserably.
"It's vulnerable, you know, writing, co-writing, telling a story and directing it. I mean I had no idea what was going to happen. I just knew that I had to try. So, the fact that it has been well received, I just wanna savour it."