2. Serena Williams, United States

Total earnings: $20.5 million

Prize money: $8.5 million

Endorsements: $12 million

At 31 years old, Williams became the oldest woman to ever hold the No 1 ranking in the world in February. It is the sixth time she has held the top spot during her decorated career. Williams has cut back her endorsement commitments in recent years, but still pitches for Nike, Wilson, Gatorade and OPI. She also has equity deals with HSN, Sleep Sheets, Mission and the Miami Dolphins.

Serena Williams of the US poses with her trophy near the Eiffel Tower in Paris after winning against Maria Sharapova of Russia in their women's singles final match at the French Open tennis tournament in Paris, June 8, 2013. [Photo/Agencies]