A spokesman for the concert organizers, Zlakto Nedanovski, said that the singer was rushed back to the United States for medical treatment.
"As she walked down the steps, she passed out. She was taken immediately to her hotel where a doctor was waiting," Nedanovski told The Associated Press.
Minnelli was advised to fly to the U.S. by her doctors there, the AP said.
"They decided together with her management that it's best for Liza to go to the U.S. right away," Nedanovski said, adding he did not know what her medical diagnosis was.
"She had felt a bit dizzy before she got up on stage," but decided to proceed with the show anyway, Nedanovski said.
The show Minnelli was performing in is called “A Classic Christmas Night."
Minnelli won an Oscar and a Golden Globe for her portrayal of Sally Bowles in the 1972 film "Cabaret." She won an Emmy for the 1972 TV special "Liza with a Z." She also won Tony Awards in 1974 and 1978.