Article continues below advertisementTennis legend Björn Borg has revealed his secret cancer battle at the age of 69.In the famed athlete's upcoming autobiography Heartbeats, which is set to be released on Tuesday, September 23, Borg details his prostate cancer diagnosis.The memoir will discuss how Borg underwent surgery to have his prostate removed after his cancer had worsened, People reported on Thursday, September 4.Article continues below advertisementBjörn Borg Exposes His Health Woes in New MemoirPer the news outlet, Borg doesn't go into much further detail about his cancer battle.