Kurt Gutmann, 82, has been called as a prosecution witness against Demjanjuk, who is accused of being an accomplice to the murder of 27,900 Polish Jews during World War II, The Times of London reported Wednesday.
Demjanjuk, 89, is being held in Munich while Germany prepares for what is set to be its last big public reckoning with Third Reich war crimes.
"As soon as I heard that Demjanjuk would be tried in Germany, I was determined to be a co-plaintiff," Gutmann says. "I want to stay alive long enough to see John Demjanjuk get his just desserts."
German authorities are in a rush to get the trial under way since the defendant and most of the witnesses are elderly and infirm.
The trial is likely to begin in mid-October, The Times says.