In her High Court trial of the murdered activist, Madikizela-Mandela claimed that she was in Brandfort in the Free State when the assault took place.
However, according to newspaper reports, medical records at the surgery of slain Soweto doctor Abu-Baker Asvat showed that Madikizela-Mandela was in fact in Johannesburg.
Sisulu, a former receptionist for Asvat, told a Truth and Reconciliation Commission hearing on Monday that the handwriting appearing in the records of former Mandela United Football Club member Katiza Cebekhulu was not hers, as had previously been claimed.
Madikizela-Mandela claims that close friend Xoliswa Falati took Cebekhulu to Asvat's surgery on December 29 1988 for proof that he had been sodomised and that she (Madikizela-Mandela) left for Brandfort that evening, returning only on December 31.
However, the medical records, which are now in the possession of the TRC, show that Cebekhulu was taken to Asvat's surgery on December 30.
Sisulu's testimony to the TRC's special hearing into the activities of the Mandela United Football Club contradicted her recent television interview with BBC journalist and author Fred Bridgland.
In the interview she confirmed that her handwriting appeared on Cebekhulu's medical card dated December 30.
Under cross-examination, Sisulu said she must have been shown a different card, because the handwriting on Cebekhulu's medical card was definitely not hers.
She also claimed that she had never seen Cebekhulu or Falati at the surgery.
"I never saw those two that day."
She also denied having ever seen Madikizela-Mandela at the surgery. Even if Madikizela-Mandela had visited the surgery, Sisulu said she would not have been able to speak to her as both were banned persons at the time.
According to Bridgland's book "Katiza's Journey", Sisulu was present at the surgery when Asvat had a "volcanic row" with Madikizela-Mandela on the day he was assassinated.
Asked if she had heard the alleged row, Sisulu told the hearing: "I would not have heard a word. My office is in the back."