Stan Wawrinka of Switzerland hits a return to Roger Federer of Switzerland during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/ANDREW GOMBERT

Roger Federer of Switzerland serves to Stan Wawrinka of Switzerland during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/RAY STUBBLEBINE

Stan Wawrinka of Switzerland hits a return to Roger Federer of Switzerland during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JASON SZENES

Roger Federer of Switzerland hits a return to Stan Wawrinka of Switzerland during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JOHN G. MABANGLO

Stan Wawrinka of Switzerland hits a return to Roger Federer of Switzerland during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/RAY STUBBLEBINE

Roger Federer of Switzerland hits a return to Stan Wawrinka of Switzerland during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JASON SZENES