The Price Tag hitmaker was hospitalised last month (Jun15) and underwent two operations to treat an unknown condition, but she confirmed on Sunday that she would be performing at the event even though she was feeling "very fragile".
After taking to the stage, she told the audience, "I didn't think I was gonna make it today and I got told not to be here, but... I can't miss shows in London."
She also joked about going against her doctor's advice in a post on Twitter.com, writing, "I know my doc is going to call me... 'Hi Jessica how are you... Resting?' 'Yes doc'".
Jessie also shared pictures of her performance on Instagram.com and wrote in the caption, "I felt so lucky to be on stage... If you had seen me last week... No one in my friends and family thought I would bounce back for this show (including myself)... It took everything I had but I did it... Thank you."