When the Williams sisters came onto the scene, they took the tennis world by storm. Serena holds the most major titles in singles, doubles, and mixed doubles combined amongst active players, male or female. Two times, she has accomplished the "Serena Slams" (winning all four majors in a row) for two consecutive years and she won several gold medals in the Olympics.
The other three are pretty special tennis stars too. Jennifer entered competition at the age of 13+ and broke many "youngest-ever" records. Lindsay, named rookie of the year in '93, is one of five women who have been the year-end World No. 1 player at least four times. Justine was forced to reitre in 2011 due to an elbow injury, but was considered one of the greatest tennis players ever, and was named as one of the "30 Legends of Women's Tennis: Past, Present and Future" by Time in 2011