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.

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.