Security guards stand outside the Olympic Tennis Centre in the Olympic Park in Rio de Janeiro, Brazil, 07 August 2016. EPA/BERND THISSEN

Viktor Troicki of Serbia in action against Andy Murray of Great Britain during their men’s singles first round match at the Rio 2016 Olympic Games Tennis events at the Olympic Tennis Centre in the Olympic Park in Rio de Janeiro, Brazil, 07 August 2016. EPA/MICHAEL REYNOLDS

Rafael Nadal of Spain in action against Federico Delbonis of Argentina during their men’s singles first round match at the Rio 2016 Olympic Games Tennis events at the Olympic Tennis Centre in the Olympic Park in Rio de Janeiro, Brazil, 07 August 2016. EPA/MICHAEL REYNOLDS

Serena Williams of the USA gets up after falling during her first round match against Daria Gavrilova of Australia at the Rio 2016 Olympic Games Tennis events at the Olympic Tennis Centre in the Olympic Park in Rio de Janeiro, Brazil, 07 August 2016. EPA/ZSOLT CZEGLEDI

Angelique Kerber of Germany in action Mariana Duque-Marino of Colombia during their first round match at the Rio 2016 Olympic Games Tennis events at the Olympic Tennis Centre in the Olympic Park in Rio de Janeiro, Brazil, 07 August 2016. EPA/MICHAEL REYNOLDS

Dustin Brown of Germany in action against Thomaz Bellucci of Brazil during their men’s singles first round match of the Rio 2016 Olympic Games Tennis events at the Olympic Tennis Centre in the Olympic Park in Rio de Janeiro, Brazil, 07 August 2016. EPA/MICHAEL REYNOLDS