Happy 21st birthday, Sloane Stephens! Ranked No. 16 in the world, Stephens turned pro in 2009 and rose to mainstream prominence when she beat fellow American Serena Williams in the quarterfinals at the 2013 Australian Open. Since her breakthrough moment, Stephens has made headlines for both her on-court play and her highly sensationalized feud with Williams after an interview she did with ESPN The Magazine went viral in May of last year. While she has yet to reach a final of a WTA event, she has netted more than $2 million in career earnings. SportsPro Media named her ninth on their 2013 list of the "World's Most Marketable Athletes" and she has deals with Under Armour, Listerine and American Express, among others. Stephens is currently competing at the Sony Open in Miami.