"US Open 2014 – Roger Federer: 'The difference between 142mph and 147mph, there's none really. Once you pass the 135 everything is just really fast'". www.ubitennis.net. August 30, 2014.
"In Their Own Words – Federer, Wawrinka, Anderson, Halep, Azarenka, Pennetta and Kvitova". U.S. Open Press Conference, www.tennispanorama.com. September 9, 2015.