Keith Urban Mentions 'Relationship Sacrifices' in Interview Months Before Nicole Kidman Split

Keith Urban's a heavyweight "Fighter" in the music industry ... and, he says the toll of being a big-time star is all-encompassing -- even when it comes to relationships.

The singer-songwriter sat for an interview with "Entertainment Tonight" months before his estranged wife, Nicole Kidman, filed for divorce ... but it seems he was already reflecting on all the sacrifices he's had to make for the music.

The interviewer asks him about financial and physical sacrifices ... and, KU offers up "relationship sacrifices" ... though he doesn't elaborate here on exactly what he means.

The comment might seem innocuous in a vacuum ... but, 'ET' dusted the interview off and published it this week ... 'cause it's obviously taken on a lot more meaning since we broke the story he and Nicole decided to call it quits.

As you know ... Keith and Nicole separated during the summer -- our sources say it was Keith's decision while Nicole still wanted to make it work -- and, NK filed for divorce a day or so after the news broke.

Keith's back on tour now ... though he's changed lyrics in at least one of his songs related to Kidman. Social media sleuths blamed Maggie Baugh -- a fill-in guitarist on Urban's tour earlier this year -- for the split, claiming there might be something romantic between them ... though neither of them have come out publicly and admitted they're togeher.