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Keith Urban had to cancel a show suddenly last night due to illness ... but there is no way in hell he is missing tonight's homecoming show, TMZ has learned.

Here's the deal ... Keith's "High and Alive" tour stop at Bon Secours Wellness Arena in Greenville, South Carolina was scrapped at the last minute yesterday ... 'cause his doctor diagnosed him with laryngitis. Naturally, the cancellation made headlines due to the split drama with Nicole Kidman -- but this had nothing to do with that. Getty

A source close to Keith tells TMZ ... the country music star is on vocal rest all day ... right up until he takes the stage Friday evening for his homecoming show at the Bridgestone Arena in Nashville. But it's totally on -- and good thing, too ... 'cause it's damn-near

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