Police and local prosecutors later said in a statement that they were investigating a case of attempted manslaughter and grievous bodily harm but that the suspect, who they did not name, had been released.
Ullrich's attorney was not immediately available for comment.
The police spokeswoman said Ullrich, 44, was under the influence of alcohol and drugs when police detained him, and that he and the woman had an argument in the Villa Kennedy hotel in the early hours of Friday morning.
According to the police statement, the woman provided extensive details about the attack while the suspect remained silent.
Ullrich told Germany's Bild newspaper in an interview this week that he had marriage problems and was emotionally stressed after his wife had decided to separate. He had not seen his three sons since Easter.
"I've done and taken things that I very much regret", Ullrich told Bild, adding: "Out of love for my children, I'm doing therapy now."
The 1997 Tour de France winner had flown to Frankfurt from his home in Mallorca, where he had been involved in an altercation in the garden of his neighbour, German action film star Til Schweiger, last weekend.
(Reporting by Michael Nienaber and Andrea Shalal; Editing by Peter Graff and Hugh Lawson)