Venus and Serena Williams
Sisters Venus(L) and Serena (R) Williams have both been ranked World's No. 1. The two are no strangers to sparing off across the net from one another, having matched up over twenty times. Pictured: Venus and Serena arrive on the red carpet at the 2015 Sports Illustrated Sportsperson of the Year Ceremony at Pier Sixty at Chelsea Piers in New York City on December 15, 2015. Photo by Dennis Van Tine/UPI