The pop superstar hit headlines in December (15) after successfully asking a New York judge to order her 15-year-old son Rocco to return from London, where he has been spending time with his filmmaker dad.

The teenager had reportedly refused to fly back to the Big Apple, where he lives with his mother, for Christmas, and had expressed an interest in living permanently in the U.K., but the judge declared Rocco must first go back to New York before a decision on future custody could be hashed out between his parents.

Madonna has yet to comment directly on the case, but she appeared to open up about the custody fight as she resumed her Rebel Heart World Tour in Mexico City.

"We all go through challenging times in our life, and that includes me," she candidly began as she performed the first of two shows at the Palacio de los Deportes venue on Wednesday. "I, too, go through challenging times in my life, and right now is one of them."

"Sometimes I think I can't do a show; I don't have the energy, I don't have the strength," Madonna continued as she wiped away a tear while her fans began chanting her name in support. "I come out here tonight and I can see your faces, and I look into your eyes and I see so much love and it gives me strength. So thank you. You are rebel hearts. I feel your hearts beating with mine."

Madonna and Guy divorced in 2008 after almost eight years of marriage, during which they adopted another son, 10-year-old David Banda.

The Material Girl star is also mum to daughters Lourdes Leon, 19, and Mercy James, nine, while Guy has three young children with his new wife Jacqui, who he wed in July (15).