One Direction - Niall Horan Struggling With Illness On One Direction Tour
The singer struggled with his vocals during the pop group's show in Denver, Colorado on Wednesday night (24Jul13) after coming down with a sore throat.
He apologised to fans for his performance, writing in a post on Twitter.com, "I apologise now to anyone comin (to) Denver show! Throat is all over the place!"
Horan later returned to Twitter on Thursday morning (25Jul13) to reveal he is still battling ill health, adding, "Woke up at 6am! Can't get back (to) sleep! It's s**t being sick isn't it?"
The band is due back on stage in Salt Lake City, Utah on Thursday night.