Roger Federer (4th L) of Switzerland, Novak Djokovic (3rd R) of Serbia, Caroline Wozniacki (4th R) of Denmark, Milos Raonic (2nd L) of Canada pose for photos with Marvel's characters during an exihibition match on Kids Tennis Day, held at Melbourne Park in Melbourne, Australia, Jan. 13, 2018. (Xinhua/Bai Xuefei)