Switzerland’s Roger Federer challenges the umpires decision during the ATP World Tour Finals semi final match against Switzerland’s Stanislas Wawrinka at the O2 Arena in London, Britain, 15 November 2014. EPA/ANDY RAIN

Switzerland’s Roger Federer returns to Switzerland’s Stanislas Wawrinka during the ATP World Tour Finals semi final match at the O2 Arena in London, Britain, 15 November 2014. EPA/ANDY RAIN

Switzerland’s Roger Federer reacts during his match with Switzerland’s Stanislas Wawrinka during the ATP World Tour Finals semi final match at the O2 Arena in London, Britain, 15 November 2014. EPA/ANDY RAIN

Switzerland’s Roger Federer returns to Switzerland’s Stanislas Wawrinka during the ATP World Tour Finals semi final match at the O2 Arena in London, Britain, 15 November 2014. EPA/ANDY RAIN

Switzerland’s Roger Federer reacts during his match with Switzerland’s Stanislas Wawrinka during the ATP World Tour Finals semi-final match at the O2 Arena in London, Britain, 15 November 2014. EPA/ANDY RAIN

Switzerland’s Roger Federer reacts after winning a point during his match with Switzerland’s Stanislas Wawrinka during the ATP World Tour Finals semi final match at the O2 Arena in London, Britain, 15 November 2014. EPA/ANDY RAIN