The 46-year-old singer and "American Idol" judge serenaded his wife on Wednesday to celebrate their eighth wedding anniversary.
The 47-year-old actress was in the audience at Urban's sold-out concert in Melbourne when he opened up about his love and performed "Once in a Lifetime," a song he wrote for Kidman during their engagement.
"A couple of months before our wedding we were in our hotel and we were talking about the future. Nic was getting very nervous about the thought of marrying me," the musician shared.
"She was saying things like 'it's a long shot' ... I knew she loved me, but I knew she was scared. I was trying to find any way I could to reassure her that I was going to be her man forever."
Urban later performed "Making Memories of Us" for his encore.
"I sang this at our wedding eight years ago," he revealed. "Eight years, baby girl! This song is for you. I love you so much."
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