The German aristocrat, a distant relation of Britain's queen, said through his lawyer at his retrial in Germany that he gave Josef Brunlehner "two symbolic slaps" because he was annoyed about the noise and laser show at the hotel disco.
"One for the music and one for the light," the 55-year-old said.
But he said that accusations that he then assaulted the 60-year-old German under the influence of alcohol were "grotesque inventions and lies."
At an earlier trial in 2004, the prince was found guilty of aggravated assault and was fined €445,000 ($616,000).
He was granted a new trial after years of legal wrangling after Princess Caroline testified before the court behind closed doors and confirmed his testimony.
News weekly Der Spiegel reported last week that other witnesses and evidence had emerged confirming that Ernst August had only slapped his victim and not hospitalised him as Brunlehner had alleged.
The trial was expected to last several days, with Brunlehner, Princess Caroline and Kenyan doctors due to give evidence.