It's good to be king. Undefeated boxing superstar Floyd Mayweather Jr., fresh off his unanimous decision win over Robert Guerrero earlier this month in Las Vegas (watch it here), is the highest-earning athlete in American sports for the second straight year. "Money" is expected to earn $90 million in 2013 according to Sports Illustrated's annual list released Wednesday (via Yahoo! Sports). Basketball star LeBron James ranks a distant second at $56.5 million. Photo by Chris Trotman for Getty Images.