Lance Armstrong managed to "mesmerise" Oprah Winfrey with his answers but did not come clean in the way she had expected, the chatshow host says.

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Speaking after she had conducted an interview with the disgraced former cyclist, Ms Winfrey said she thought that all "the most important questions and the answers that people around the world have been waiting to hear were answered".

But she added that having prepared for the interview as if it were "a college exam", she said she was "surprised" by him, and that he was thoughtful, serious and well-prepared.

Earlier, USA Today reported that Armstrong told Ms Winfrey that he started using performance-enhancing drugs to gain an edge in cycling in the mid-1990s before he was diagnosed with cancer.

She told CBS' This Morning show: "I would say that he did not come clean in the manner that I expected. It was a surprise to me.

"For myself, my team, all of us in the room, we were mesmerised and riveted by some of his answers."

She added: "I had prepared and prepared like it was a college exam, and walked into the room with 112 questions.

"In a two-and-a-half hour interview I asked at least as many of those questions as I could, but I felt he answered the questions in a way that he was ready."

Ms Winfrey appeared on CBS This Morning on Tuesday to discuss the interview with Armstrong, who was stripped of his seven Tour de France titles for drugs offences.

After conducting the interview on Monday night, Ms Winfrey tweeted: "Just wrapped with @lancearmstrong More than 2 1/2 hours . He came READY!"

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A report by the US Anti-Doping Agency accused the 41-year-old of taking drugs and of masterminding the "most sophisticated, professional and successful doping program that sport has ever seen". The cyclist had always professed his innocence, but has lost millions of dollars in endorsement and sponsorship deals, as well as legal challenges including from the Sunday Times.

USA Today reported that a "person said to be familiar with the situation" had said that Armstrong had admitted cheating, but the person spoke on condition of anonymity.

Before the interview was recorded, Armstrong made a formal apology in person to the staff of Livestrong cancer foundation.

Armstrong set up the foundation in 1997 following his own recovery from cancer but stood down as chairman in October last year "to spare the foundation any negative effects as a result of controversy surrounding (his) cycling career".

Charity spokesperson Katherine McLane was quoted as saying Armstrong had made a "very sincere and heartfelt apology to the staff".