Children hug and play with a giant tennis ball during childrens day at the Melbourne Park grounds prior to the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 17 January 2015. The Australian Open tennis tournament will go from 19 January until 01 February 2015. EPA/FILIP SINGER

Ana Ivanovic (C) of Serbia, Roger Federer (R) of Switzerland and Victoria Azarenka of Belarus (L) smile as they perform during the Kids Tennis Day ahead of the Australian Open Grand Slam tennis tournament in the Rod Laver Arena, Melbourne Park, in Melbourne, Australia, 17 January 2015. The event is watched by thousands of children.The Australian Open tennis tournament runs from 19 January until 01 February 2015. EPA/FILIP SINGER

Australian tennis player Nick Kyrgios and a ninja turtle during the kids day at the Australian Open practice session at Melbourne Park in Melbourne, Australia, 17 January 2014. The Australian Open tennis tournament runs from 19 January until 01 February 2015. EPA/DAVID CROSLING AUSTRALIA AND NEW ZEALAND OUT

Roger Federer (L) of Switzerland and Victoria Azarenka of Belarus chat as they perform during the Kids Tennis Day ahead of the Australian Open Grand Slam tennis tournament in the Rod Laver Arena, Melbourne Park, in Melbourne, Australia, 17 January 2015. Event is watched thousend of childerns.The Australian Open tennis tournament runs from 19 January until 01 February 2015. EPA/FILIP SINGER

A handout picture released by Tennis Australia shows Mirka Federer (C), wife of Roger Federer of Switzerland, and their two sets of twins, the boy babies Lenny and Leo at back and the girls Myla Rose and Charlene Riva (C-F) and Mary Jo Fernandez (R- back) and Federer’s coach Stefan Edberg (2-L); watch as Federer plays on court with other players during the Australian Open 2015 Kids Tennis Day in the Rod Laver arena prior the Australian Open Grand Slam tennis tournament in Melbourne, Australia, 17 January 2015. The Australian Open tennis tournament will run from 19 January until 01 February 2015. EPA/FIONA HAMILTON HANDOUT EDITORIAL USE ONLY

Roger Federer of Switzerland performs with a mascot during the Kids Tennis Day ahead of the Australian Open Grand Slam tennis tournament in the Rod Laver Arena, Melbourne Park, in Melbourne, Australia, 17 January 2015. Event is watched thousend of childerns.The Australian Open tennis tournament runs from 19 January until 01 February 2015. EPA/FILIP SINGER

(L-R) Roger Federer, from Switzerland, Ana Ivanovich, of Serbia, Victoria Azarenka of Belarus, Eugenie Bouchard of Canada, Nick Kyrgios and Thanasis Kokkinakis both from Australia, pose during the Kids Tennis Day ahead of the Australian Open Grand Slam tennis tournament in the Rod Laver Arena, Melbourne Park, in Melbourne, Australia, 17 January 2015. The event is watched thousand of childern.The Australian Open tennis tournament runs from 19 January until 01 February 2015. EPA/FILIP SINGER