The 45-year-old singer, who filed for divorce from the British rocker, 49, last week after 13 years of marriage, attempted to put her personal troubles aside to film the season nine battle rounds for the NBC reality series in Los Angeles, reported Us magazine.
"Gwen seems sad," an eyewitness said.
The former couple, who have three sons, Kingston, nine, Zuma, six, and Apollo, 17 months, together, are said to have been unhappy for some time.
"It only looks sudden, but everything was worked out months ago so paperwork could be filed and the marriage could be ended quickly and easily.