World No. 9 debuts shorter do.
Top players from Maria Sharapova to Garbine Muguruza staying sharp by boxing.
"Very happy it's happening right now and can't wait to get back," Federer says of his January return at the Hopman Cup.
Hall of Famers predict world No. 8 will make Grand Slam breakthrough.
"It's just always going to be Justin Bieber," says world No. 2.
The event will feature the top 7-ranked 21-and-under players in the world, plus a wild card.
The doubles final is set in London, and the No.1 rankings are locked up as well.
