Ivor Perl broke off his testimony at what is likely to be one of the last trials held over the crimes of the Nazis, to directly address the defendant, Oskar Gröning.
"I want to say something to the defendant," Mr Perl said."You know, Oskar, I don't want to call you Mister.
"When I was asked if I'd like to testify, I said I'm not doing that. I was afraid to come here and look at you. Now I'm sitting here and I see someone who is sorry.
"I'm sorry too, that I was afraid, that I worried, that I wasted energy, that I had sleepless nights because of you."
The 93-year-old Mr Gröning said nothing, but remained looking down with his arms crossed.
Earlier, Mr Perl, who lives in Buckhurst Hill, Essex, told the court how he arrived at Auschwitz at the age of 12, and only survived because he successfully pretended to be 16.
"I'll try to stick to the facts," Mr Perl said. "But to me this is so important that I have to include personal feelings. Please forgive me."
The 83-year-old told the court how he was born in Hungary into a religious Jewish family.
"My recollection is of a happy, normal life, as far as that went in fascist times," he said. "Normal was, we [were] spat on and attacked."
His family was forced to live in a ghetto. Later they were told they would be transported to the east, and given farmland.
"We thought anything would be better than the ghetto," Mr Perl said.
But the family was taken to Auschwitz. When they arrived, Polish Jews working on the train tracks shouted out to them, warning the children to pretend to be 16.
The 12-year-old Mr Perl had no idea that the warning would save him from the gas chambers.
At the entrance to the camp, he wanted to go with his mother, but they were separated by the SS. It was the last time he saw his mother and sisters alive.
The new arrivals who were allowed to live were given a concentration camp number.
"'You're in Auschwitz now,' they said," Mr Perl told the court. "'You get a number. Never forget it, or you don't exist any more.'"
Mr Perl could still remember the number by heart, he said.
Mr Perl lost his parents and seven siblings during the Holocaust. Only he and his brother survived. He spent a year in Auschwitz and various other concentration camps before he was liberated from Dachau in May 1945.
Mr Gröning asked forgiveness at the start of the trial last month, admitting he knew that Jews were being sent to the gas chambers but insisted he was never involved in killing them.
"For me, there's no question that I share moral guilt," he told the court. "I ask for forgiveness. Whether I am guilty under criminal law, you will have to decide."
Before the trial he was one of few former SS members to speak out publicly about the crimes of the Holocaust.
The trial is the first to take place since a key ruling in 2011, when former SS guard John Demjanjuk was found guilty of being an accessory to the murder of 27,900 Jews at the Sobibor extermination camp.
The ruling overturned years of German legal precedent that only the senior Nazi leadership could be held responsible for the Holocaust.
The trial continues.