The incident happened at the Mercedes Sosa theatre in Tucuman, a city in north-west Argentina.
Footage showed Denis, real name Héctor Omar Hoffmann, plunging several feet into an open orchestra pit to the horror of fans watching him perform who tried to reach him as they urged others to call for an ambulance.
His girlfriend Veronica Monti revealed afterwards he had called her minutes before starting his Monday night show to say he was tired and felt like going to bed instead of doing his show.
She added: "We are hoping for a miracle. He speaks with God every night and thinks God listens to him. We’re hoping to have better news later today."