The Australian songwriter performed her hit Chandelier at the Friends star's wedding reception, which was held in August (15) at the couple's California mansion under the guise of a birthday party.
She now admits her rendition did not go smoothly as her vocal cords failed her on the high notes in front of a celebrity audience which included Courteney Cox, Tobey Maguire, Emily Blunt and John Krasinski.
"I sang at my friend's wedding a couple of weeks ago and it was actually just laughable," she told Australian radio hosts Fitzy and Wippa on NOVA FM. "I went for the Chandelier chorus and it was like, it was actually hilarious. It was squawking... It sounded like three weird squeaky notes at the same time were coming out... it was hilarious."
When asked who the famous friend was, Sia replied, "You guys can probably guess, I only have a couple of famous friends... it was all over the papers."
She also recalls fumbling with her vocals while performing Titanium, her collaboration with David Guetta, at a Presidential gala, adding, "I totally couldn't hit the high note in front of the President! I think I did three songs and I think the end was Titanium and I just couldn't even hit that high note by the end. It was hilarious... I opened my mouth and a squeak came out."
During the radio interview, she also admitted she rarely warms up before performing and she has been warned she will lose her voice if she continues.
"I don't warm up, I've never... that's a lie, I had a couple of singing lessons right before the (2015) Grammys because I lost my voice and I couldn't hit the high notes and everyone was saying there's this guy, he will help you condition your voice so that you can hit the high notes. All these exercises, it was so crazy. It was $500 an hour, that's crazy, I can already sing. But he said if you keep singing the way you sing you will lose your voice like most people do."