Hanif Vally, the Truth and Reconciliation Commission investigator, seemed completely flabbergasted. "I wasn't expecting this," he said, fumbling for his next question. Neither - with perhaps a few notable exceptions - was anyone else.
Yesterday was the sixth day of hearings into allegations that Mrs Mandela and her notorious Mandela United Football Club committed at least six murders and an array of assaults in the late 1980s.
The evidence has piled up to support the theory that Mrs Mandela murdered Soweto doctor Abu-Baker Asvat in January 1989 because he knew too much about her alleged involvement in the murder of a 14-year-old activist, Stompie Seipei Moeketsi, four weeks previously.
But Mrs Sisulu yesterday knocked a hole in that one. She completely withdrew the information that damned Mrs Mandela, which she recently provided for the BBC documentary Katiza's Journey.
Mrs Sisulu, the nurse in Dr Asvat's surgery, yesterday denied that the writing on a crucial patient record card was hers. If verified, the card, belonging to Katiza Cebekulu - the so-called missing witness in the Winnie scandal - would have completely undermined Mrs Mandela's alibi in the Stompie case.
There was more disappointment for Dr Asvat's family and Stompie Seipei's mother, Joyce, who have put their faith in the TRC. Mrs Sisulu said she had no knowledge of a "volcanic row" alleged to have erupted between Mrs Mandela and Dr Asvat hours before his murder.
Mrs Sisulu gave an emotional account of how she heard two shots and Dr Asvat's scream before finding him lying in his consulting room bleeding from his chest and gasping for breath. Mrs Sisulu claimed she and the doctor were like mother and son. But she said that if he had trouble with Mrs Mandela he never told her.
Like other ANC figures who have testified, Mrs Sisulu seemed evasive. She was unable to explain the discrepancy between what she told the BBC and her evidence yesterday.
TRC Commissioner Dumisa Ntsebeza was the only one with the courage to take on an icon. He suggested Mrs Sisulu, like other ANC members, was hedging on anything that would implicate Mrs Mandela.
"Is it because the Mandela and Sisulu families have come along way together...?" he asked, referring to her husband Walter's lifelong journey with President Nelson Mandela, and her own friendship, long ago soured, with Mrs Mandela. "Is it because you wouldn't like to be the one identified in South African history to speak about your comrade ... as being involved in the death of Dr Asvat?"
Mr Ntsebeza was rewarded with tears and angry indignation. Mrs Sisulu reeled off her long years of sacrifice for the movement. "I am not here to tell lies," she said bitterly. Even Mr Ntsebeza had not the stomach to continue.
Yesterday's hearing was marred by many allegations - from Commission investigators and those called to testify witnesses - that Mrs Mandela was intimidating witnesses. TRC investigators have been told by potential witnesses that they have been visited by "Mama" or called to her Soweto home. One of those witnesses was last night proving "difficult to locate."
At the end of yesterday's hearing few would have hazarded a guess as to the behind-the-scenes political machinations. If the ANC leadership is hanging Mrs Mandela out to dry - it opposes her standing for deputy leadership of the party later this month - why did the ANC's Mathews Phosa, the first senior member to show support, turn up yesterday?
More galling still is the question why did he choose to kiss Mrs Mandela immediately after a tearful witness claimed Mrs Mandela had ordered Dr Asvat's murder. The Asvat family were still weeping when the embrace took place.