Ana Ivanovic of Serbia hits a return to Dominika Cibulkova of Slovakia during their match on the first day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Ana Ivanovic of Serbia hits a return to Dominika Cibulkova of Slovakia during their match on the first day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Ana Ivanovic of Serbia reacts as she plays Dominika Cibulkova of Slovakia during their match on the first day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Ana Ivanovic of Serbia reacts as she plays Dominika Cibulkova of Slovakia during their match on the first day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 31 August 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/DANIEL MURPHY