The 30-year-old reality show judge and former Destiny's Child star apparently refused to let her four finalists have just four hours off in between finishing filming the judges' house stage of the contest and their flight back from Miami to the UK.
According to The Sun, she decided not to waste a second and ordered the girls to attend an intensive four-hour vocal training session with America's top singing teacher Rob Stevenson.
A source said, 'They were ready to celebrate but Kelly had other plans. Rob really put them through their paces.'