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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. Gallery Launch Gallery Getty

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 mean

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