Lance Armstrong beat the odds many times. The seven-time Tour de France winner emerged victorious not only in a number of difficult bicycle races, but he also survived stage three testicular cancer. But when the 40-year-old confronted his most recent challenge, he decided not to continue any further. 

Armstrong announced Thursday night that he wanted to discontinue fighting allegations that he used banned performance-enhancing drugs during his sporting career. His announcement preceded the U.S. Anti-Doping Agency's decision to impose a lifetime ban on the champion cyclist and strip him of his seven Tour de France titles.

However, Armstrong maintained his innocence and called the USADA's case a "witch hunt."

"I have been dealing with claims that I cheated and had an unfair advantage in winning my seven Tours since 1999. Over the past three years, I have been subjected to a two-year federal criminal investigation followed by Travis Tygart's unconstitutional witch hunt. The toll this has taken on my family, and my work for our foundation and on me leads me to where I am today -- finished with this nonsense," Armstrong said in a statement.

According to Travis Tygart, chief executive of the USADA, the agency has the right to take away the titles from Armstrong, one of the greatest cyclists in history. The agency said that Armstrong had used banned substances since 1996, which included the blood-booster EPO and steroids, as well as blood transfusions.

"It is a sad day for all of us who love sport and our athletic heroes," the Washington Post quoted Travis Tygart, chief executive of the USADA, as saying. "This is a heartbreaking example of how the win-at-all-costs culture of sport, if left unchecked, will overtake fair, safe and honest competition."

However, Armstrong said that it was not the USADA, but the International Cycling Union, which runs the Tour de France, that has the authority to make such a decision.

"USADA cannot assert control of a professional international sport and attempt to strip my seven Tour de France titles. I know who won those seven Tours, my teammates know who won those seven Tours, and everyone I competed against knows who won those seven Tours," said Armstrong.

Armstrong, who announced his retirement last year, said that he would no longer fight the doping charges as he was tired of doing so. He also pointed to the hundreds of drug tests that he claimed he passed as proof of being innocent.

"Today I turn the page. I will no longer address this issue, regardless of the circumstances. I will commit myself to the work I began before ever winning a single Tour de France title: serving people and families affected by cancer, especially those in underserved communities," Armstrong said.

Read Lance Armstrong's full statement here.