The One Thing hitmakers were belting out a rendition of the 2000 tune at the Motorpoint Arena Cardiff on Saturday (02Mar13), when Tomlinson suffered a little memory loss.
The red-faced 21 year old told the crowd, "Ok, I just forgot the lyrics. I'm not even going to pretend I know them."
His confession prompted his bandmates to step in and help him out, and Tomlinson was able to carry on with the song without incident, later apologising to fans, stating, "I'm sorry, Cardiff."
However, it appears the embarrassing moment didn't put a damper on the rest of Tomlinson's night - taking to his Twitter.com blog, he wrote, "What a great gig ! Loved that !!!!"