U.S. Open semifinals

Marin Cilic celebrates after defeating Roger Federer.  (Streeter Lecka / Getty Images)

Marin Cilic celebrates after defeating Roger Federer.  (MIKE SEGAR / Reuters Photo)

Marin Cilic hits a return to Roger Federer.  (KENA BETANCUR / AFP/Getty Images)

Roger Federer returns a shot against Marin Cilic.  (Matthew Stockman / Getty Images)

Novak Djokovic shakes hands with Kei Nishikori.  (Matthew Stockman / Getty Images)

Kei Nishikori celebrates his victory over Novak Djokovic.  (TIMOTHY A. CLARY / AFP/Getty Images)

Kei Nishikori reacts.  (Julian Finney / Getty Images)

Novak Djokovic serves against Kei Nishikori.  (Mike Stobe / Getty Images for USTA)

Denmark’s Caroline Wozniacki of celebrates after winning her U.S. Open semifinal match against China’s Peng Shuai, whose injury forced her to retire from the match.   (Adam Hunger / Reuters Photo)

Peng Shuai kneels and as play stops at the U.S. Open.   (Timothy A. Clary / AFP / Getty Images)

Peng Shuai receives medical treatment.  (Robert Deutsch / Reuters )

Peng Shuai is tended to after she dropped to the court in pain as her opponent Caroline Wozniacki of Denmark reaches out to her.  (Mike Segar / Reuters)

Caroline Wozniacki checks on her opponent, Peng Shuai.   (Adam Hunger / Reuters)

Peng Shuai grimaces as she is placed in a wheelchair.   (Mike Segar / Reuters)

Peng Shuai reacts after being placed in a wheelchair.   (Mike Segar / Reuters)

China’s Peng Shuai hits a return to Caroline Wozniacki.  (Mike Segar / Reuters)

Caroline Wozniacki returns the ball to Peng Shuai.  (Mike Segar / Reuters)

Caroline Wozniacki of Denmark celebrates after winning a point.   (Adam Hunger / Reuters)

Caroline Wozniacki serves to Peng Shuai.   (Adam Hunger / Reuters)

Peng Shuai reaches for a return.  (Adam Hunger / Reuters)

Caroline Wozniacki smashes a return.  (Adam Hunger / Reuters)

Caroline Wozniacki volleys with Peng Shuai.   (Adam Hunger / Reuters)

Caroline Wozniacki celebrates after winning her semifinal match.   (Adam Hunger / Reuters)