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NEW YORK, NY - SEPTEMBER 11: Serena Williams of the United States returns a shot to Roberta Vinci of Italy during their Women's Singles Semifinals match on Day Twelve of the 2015 US Open at the USTA Billie Jean King National Tennis Center on September 11, 2015 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Clive Brunskill/Getty Images)

NEW YORK, NY - SEPTEMBER 11:  Serena Williams of the United States returns a shot to Roberta Vinci of Italy during their Women's Singles Semifinals match on Day Twelve of the 2015 US Open at the USTA Billie Jean King National Tennis Center on September 11, 2015 in the Flushing neighborhood of the Queens borough of New York City.  (Photo by Clive Brunskill/Getty Images)