The 20-year-old Stephens upset No. 1-ranked Serena Williams this year at the Australian Open, but Williams went on to win two majors this season, the French Open and U.S. Open. Stephens started the year ranked No. 38 in the world and has moved up to 12th.
Isner won the 2011 Hopman Cup with Bethanie Mattek-Sands in 2011 and won with Venus Williams in 2013.
Sam Stosur, 2011 U.S. Open champion, and Bernard Tomic will represent the home country of Australia.