World No . 1 Novak Djokovic will enter this year 's U.S. Open without a title since April . He loses to big - serving American John Isner at the Cincinnati Masters on grass . Andy Murray also exits in the quarterfinals after a defeat to Czech Tomas Berdych . Serena Williams reaches the semifinals by ousting Romania 's Simon Halep .
World No . 1 Novak Djokovic will enter this year 's U.S. Open without a title since April . Djokovic loses to big-serving American John Isner at the Cincinnati Masters on Friday . Andy Murray also exits in the quarterfinals following a defeat to Czech Tomas Berdych . Serena Williams reaches the semifinals by ousting Romania 's Simona Halep .