Players at their best in US Open

Belarus's Victoria Azarenka returns a shot to Germany's Dinah Pfizenmaier during the first round of the U.S. Open tennis tournament in New York. (AP Photo)

Filippo Volandri, of Italy, returns a shot to John Isner during the first round of the 2013 U.S. Open tennis tournament in New York. (AP Photo)

John Isner returns a shot to Filippo Volandri, of Italy, during the first round of the 2013 U.S. Open tennis tournament Tuesday, Aug. 27, 2013, in New York. Isner defeated Volandri. (AP Photo)

Dinah Pfizenmaier, of Germany, serves against Victoria Azarenka, of Belarus, during the first round of the 2013 U.S. Open tennis tournament in New York. Azarenko won 6 0, 6 0. (AP Photo)

Svetlana Kuznetsova, of Russia, returns against Mallory Burdettte, of the United States, during the first round of the 2013 U.S. Open tennis tournament. (AP Photo)

In this image taken with a slow shutter speed, Novak Djokovic, of Serbia, returns a shot to Ricardas Berankis, of Lithuania, during the first round of the U.S. Open tennis tournament Tuesday, Aug. 27, 2013, in New York. (AP Photo)

Serena Williams of the United States returns a shot to Francesca Schiavone of Italy during their women's singles first round match at the U.S. Open tennis championships in New York. Serena Williams won 2 0.