Swiss Roger Federer of the Obi UAE Royals in action against Stan Wawrinka of the OUE Singapore Slammers during their match at the International Premier Tennis League (IPTL) held at the indoor stadium in Singapore, 18 December 2015. EPA/WALLACE WOON

Swiss Roger Federer of the Obi UAE Royals in action against Stan Wawrinka of the OUE Singapore Slammers during their match at the International Premier Tennis League (IPTL) held at the indoor stadium in Singapore, 18 December 2015. EPA/WALLACE WOON

Swiss Roger Federer of the Obi UAE Royals in action against Stan Wawrinka of the OUE Singapore Slammers during their match at the International Premier Tennis League (IPTL) held at the indoor stadium in Singapore, 18 December 2015. EPA/WALLACE WOON

Swiss Roger Federer (L) and French Kristina Mladenovic (R) of the of the Obi UAE Royals in action against Karolina Pliskova and Dustin Brown of the OUE Singapore Slammers during their match at the International Premier Tennis League (IPTL) held at the indoor stadium in Singapore, 18 December 2015. EPA/WALLACE WOON

Swiss Roger Federer of the Obi UAE Royals in action against Karolina Pliskova and Dustin Brown of the OUE Singapore Slammers during their match at the International Premier Tennis League (IPTL) held at the indoor stadium in Singapore, 18 December 2015. EPA/WALLACE WOON