Leon Spinks, the former undisputed heavyweight boxing champion who in 1978 defeated Muhammad Ali in one of the biggest upsets in the sport’s history, is in a Las Vegas hospital battling prostate cancer, though he's showing "small signs of improvement."

Family spokesperson Lindsay Feldman told Fox News that Spinks was diagnosed with cancer earlier this year and it has since spread to his bladder.

“The last few months have been an agonizing roller coaster for Leon and his wife, Brenda, with continuous hospital stays. Leon is showing small signs of improvement and progress,” Feldman said. “A miraculous fighter his entire life, we are optimistic and hopeful that he will move out of ICU soon. The power of prayer is real and his family is beyond grateful for all of the tremendous love and support.”

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Brenda Spinks, the wife of the boxing legend, put out a concerning message on her Facebook on Monday asking for prayers for her husband.

“Dear Friends: I know it's been a while. However, you know how I Believe in the Power of Prayer," she wrote. "It's been a tough year for us. Leon has endured a lot of medical problems. I'm reaching out to ask you to kindly pray for my Beautiful Husband Leon. So that he may overcome the obstacles that have crossed his path. We Love You All & Appreciate Your Support.”

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Spinks, 66, has battled several health issues over the last few years. He was diagnosed with brain shrinkage in 2012, a condition doctors said stemmed from the beatings he took in the ring, and in 2014 he was hospitalized for abdominal problems.

The boxing legend was known for his victory over Ali in 1978 but had his WBC heavyweight title stripped after facing Ali in an unsanctioned rematch seven months later.

Spinks also won a gold medal in the light heavyweight division at the Montreal Olympics in 1976.

He finished his career with a 26-17-3 record, including 14 wins by knockout.