MELBOURNE, AUSTRALIA - JANUARY 13:  Milos Raonic of Canada, Roger Federer of Switzerland, Caroline Wozniacki of Denmark and Novak Djokovic of Serbia, pose with Marvel characters following the Rod Laver Arena Spectacular as part of Kids Tennis Day ahead of the 2018 Australian Open at Melbourne Park on January 13, 2018 in Melbourne, Australia.  (Photo by Graham Denholm/Getty Images)