But Thomas Blatt, 52, told a court in Munich, Germany, that Ukrainians "were the worst; they were everywhere," The Times of London reported Wednesday.
"I can't say I remember Demjanjuk's face but frankly I can't even recall that of my father after so many decades," he testified.
Demjanjuk is accused of being a Sobibor guard and complicit in 27,900 murders. His trial is being billed as the last big Nazi war crimes trial.
Blatt, a Polish Jew, was a teenager when he was sent to the Sobibor camp.
He said Ukrainians were the ones who surrounded the trainloads of Jews arriving at the camp and prodded them with bayonets toward the gas chambers.
Demjanjuk, a baseball cap hiding his eyes, said nothing in court Tuesday. He was lying on a mobile sick bed.
Demjanjuk emigrated to the United States after the war and worked in the car industry near Cleveland. Authorities extradited him to Germany last May.