Caroline Wozniacki of Denmark returns a shot against Shuai Peng of China during their women's singles semifinal match on Day Twelve of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 5, 2014. (Photo by Mike Stobe/Getty Images for USTA)
2014 US Open - Day 12
Shuai Peng of China is tended to by trainers after getting injured while playing against Caroline Wozniacki of Denmark during their women's singles semifinal match on Day Twelve of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 5, 2014. (Photo by Chris Trotman/Getty Images for USTA)
2014 US Open - Day 12
Peng Shuai of China falls to her knees and stops play as she is helped by medics and Caroline Wozniacki (C) of Denmark during their 2014 US Open Women's Singles Semifinals match at the USTA Billie Jean King National Tennis Center. (Photo credit should read TIMOTHY A. CLARY/AFP/Getty Images)
2014 US Open - Day 12
Serena Williams of the United States serves to Ekaterina Makarova of Russia during their women's singles semifinal match on Day Twelve of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 5, 2014. (Photo by Al Bello/Getty Images)
2014 US Open - Day 12
Ekaterina Makarova of Russia returns a shot against Serena Williams of the United States during their women's singles semifinal match on Day Twelve of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 5, 2014. (Photo by Elsa/Getty Images)
2014 US Open - Day 12
Serena Williams of the United States celebrates after defeating Ekaterina Makarova of Russia during their women's singles semifinal match on Day Twelve of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 5, 2014. (Photo by Chris Trotman/Getty Images for USTA)
2014 US Open - Day 12
Caroline Wozniacki of Denmark hits a ball into the crowd after defeating Shuai Peng of China during their women's singles semifinal match on Day Twelve of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 5, 2014. (Photo by Mike Stobe/Getty Images for USTA)
2014 US Open - Day 12
Serena Williams of the United States celebrates after defeating Ekaterina Makarova of Russia during their women's singles semifinal match on Day Twelve of the 2014 US Open at the USTA Billie Jean King National Tennis Center on September 5, 2014. (Photo by Chris Trotman/Getty Images for USTA)
