Boyle, 48, has been touring with other finalists from the most recent season of the TV competition show.
She also backed out of two gigs in Manchester, England, earlier this week, reportedly because she was too tired to take the stage.
"Susan's really upset that she can't perform in Cardiff tonight and she's really sorry to all her fans," her agent told WalesOnline Friday. "She gets tired very easily and she needs to rest. The tour itself is a really tiring process and she would have loved to have been able to be there tonight."
She is expected to perform in Nottingham Saturday, WalesOnline said.