"We don't yet know what happened. We are all very worried," Minnelli's manager, Gery Labriola, told The Local newspaper.
Minnelli sang three songs, then dropped her microphone and tumbled off the stage, where she was caught by the show's production manager, The Local reported.
Concert promoter Mikael Silverskiold said Minnelli wasn't injured in the fall.
Spokeswoman Liz Rosenberg said in a statement that Minnelli felt dizzy before the concert started, but didn't want to drop out of the show.
"(She) became very ill shortly after the show began," Rosenberg's statement said. "As a result, she was unable to finish her performance and was rushed back to her hotel by her manager. A doctor upon examining her and in consultation with her New York physician, determined that it would be necessary for the Oscar-winning actress and singer to immediately return to New York City to receive further medical evaluation."
Minnelli reportedly flew to the United States Thursday morning, The Local said.