The Grand Rapids Press says Simpson forgot her lyrics, struggled with her ear monitors, mumbled through songs and fought back tears during Thursday night's performance.
At one point, she asked her backing band to start over on one song, and she forgot the words to another.
Cindi Berger, a representative for Simpson, says the 28-year-old singer-actress had an "off night."
Simpson apologized to the supportive audience of about 9,000 by mouthing the word "sorry" on two large video screens.
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