Gael Monfils of France hits a return to Roger Federer of Switzerland (R) during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/ANDREW GOMBERT

Roger Federer of Switzerland hits a return to Gael Monfils of France during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JUSTIN LANE

Gael Monfils of France serves to Roger Federer of Switzerland during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JUSTIN LANE

Roger Federer of Switzerland serves to Gael Monfils of France during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JOHN G. MABANGLO

Gael Monfils of France reacts as he plays Roger Federer of Switzerland during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JOHN G. MABANGLO

Roger Federer of Switzerland hits a return to Gael Monfils of France during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JUSTIN LANE

Roger Federer of Switzerland hits a return to Gael Monfils of France during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/JOHN G. MABANGLO

Gael Monfils of France hits a return to Roger Federer of Switzerland during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/ANDREW GOMBERT

Roger Federer of Switzerland hits a return to Gael Monfils of France during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/ANDREW GOMBERT

Roger Federer of Switzerland reacts after defeating Gael Monfils of France during their quarterfinals round match on the eleventh day of the 2014 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 04 September 2014. The US Open runs through 08 September, a 15-day schedule. EPA/ANDREW GOMBERT