Despite her illness, singer Taylor Swift performed at the two festivals.

The 24-year-old, who performed at the annual KIIS FM Jingle Ball at the Staples Center here Friday, warned fans ahead of the show that her voice was not at its best, reports contactmusic.com.

She wrote on her Twitter page: `I'm about to go onstage at... KIISJingleBall in La. Fun fact: I have a wicked case of laryngitis. This should be interesting!`

Despite her illness, Swift flew to London to headline the Capital FM Jingle Bell Ball Sunday (07Dec14), and issued a second warning to her fans, writing, `The voice is feeling hella (really) bad but I'm here and ready to Party/sound like a frog.`

The `Shake It Off` hitmaker also thanked fans for their support while onstage, telling them, `I do have a severe case of laryngitis which is making it hard to sing, but you're helping me... I want to thank you for staying and hanging out with me.`