Since the start of the 1999 tennis season, Serena Williams has won $56.6 million in her career on the WTA tour. Meanwhile, Roger Federer has won $79.8 million (41.2% more) playing on the ATP Tour despite a similar career.
Serena Williams played in her first Grand Slam in 1998 and has won 17 in her career. Roger Federer played in his first Grand Slam in 1999 and has won 17 in his career.
Of course, a big difference is that for much Williams career there was not equal pay at the Grand Slam events. That ended in 2007. Since then, things have evened out with Williams earning $38.8 million since the start of the 2008 season and Federer taking him $41.2 million.
BusinessInsider.com