"It seems Mr. Ullrich and an escort woman had a dispute and that he attacked her," Frankfurt police spokesperson Carina Lerch said. "She alerted the hotel staff and they called police."
Lerch said the retired German cyclist was under the influence of drugs and alcohol.
The 31-year-old woman needed medical treatment. Lerch said no further details could be given on her condition while police were investigating.
Later, a police statement said Ullrich had resisted arrest.
"As part of the police interrogation, the woman provided extensive information on the attacks of the accused. The accused so far makes use of his right to refuse to testify," the statement said.
"Currently, the accused is being investigated for attempted manslaughter and dangerous assault. The investigation has not yet been completed."
Police said Ullrich was to be released later Friday after consultation with the prosecutor's office in Frankfurt "because the conditions for detention are not met."
News agency dpa reported that the 44-year-old Ullrich had been arrested the week before in Mallorca, where he lives, after an incident on the property of his neighbor, actor Til Schweiger.
On Monday, Ullrich told German tabloid Bild of personal problems.
"The separation from (my wife) Sara and distance from my children, whom I have not seen since Easter and have barely spoken to, have had a great effect on me. I have done things as a result that I very much regret," Ullrich told the mass circulation daily.
Ullrich, the only German to win the Tour de France, was dogged by doping allegations during his career and admitted to German magazine Focus in 2013 that he received blood-doping treatment from Spanish doctor Eufemiano Fuentes.
"Almost everyone took performance-enhancing substances then. I took nothing that the others didn't also take," the 1997 Tour champion said then.
Ullrich served a two-year ban for doping in 2012, five years after he retired.
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