Tennis players make up the majority of the world's highest-paid female athletes, according to an annual list published by Forbes on Tuesday ahead of next week's US Open.
The American, who returned to competition in March after giving birth last September, earned $62,000 (£48,000) in winnings in the year through to June, but received $18.1 million (£14m) from an endorsement portfolio unmatched among women in sports, Forbes said.
Williams, who will try to equal Margaret Court's record of 24 Grand Slam titles when she competes at the US Open in New York, earned twice as much off the court as any other female athlete, according to Forbes.
Dane Caroline Wozniacki, who captured her maiden Grand Slam title at this year's Australian Open, was second on the list with combined earnings of $13m/£10.1m while reigning US Open champion Sloane Stephens ($11.2m/£8.7m) was third.
Indian badminton player P.V. Sindhu ($8.5m/£6.6m) and retired race car driver Danica Patrick ($7.5m/£5.8m) were the only non-tennis players to crack the top 10, filling up the seventh and ninth spots, respectively.