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Serena Williams reacts against Roberta Vinci during their semi-final at the 2015 US Open. (Photo by Alex Goodlett/Getty Images)

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Serena Williams during her semi-final. (Photo by Matthew Stockman/Getty Images)

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Serena Williams waves goodbye after being eliminated by Italian Roberta Vinci in the semi-finals. (Photo by Matthew Stockman/Getty Images)

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WOMEN'S FINAL: Roberta Vinci returns a forehand shot to Flavia Pennetta. (Photo by Maddie Meyer/Getty Images)

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WOMEN'S FINAL: Roberta Vinci reacts during the Women's Singles Final. (Photo by Maddie Meyer/Getty Images)

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WOMEN'S FINAL: Flavia Pennetta returns to Roberta Vinci. (Photo by Al Bello/Getty Images)

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WOMEN'S FINAL: Flavia Pennetta during the Women's Singles Final. (Photo by Matthew Stockman/Getty Images)

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WOMEN'S FINAL: Flavia Pennetta celebrates after defeating good friend Roberta Vinci. Pennetta defeated Vinci 7-6, 6-2. (Photo by Alex Goodlett/Getty Images)

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WOMEN'S FINAL: Flavia Pennetta hugs Roberta Vinci after the Women's singles final. (Photo by Clive Brunskill/Getty Images)

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WOMEN'S FINAL: Flavia Pennetta and Roberta Vinci joke after the final. (Photo by Matthew Stockman/Getty Images)

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WOMEN'S FINAL: Flavia Pennetta celebrates with the trophy after winning the 2015 US Open. (Photo by Clive Brunskill/Getty Images)

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MEN'S FINAL: A general view of pre-match ceremonies prior to the Men's Singles Final match between Roger Federer and Novak Djokovic. (Photo by Michael Heiman/Getty Images)

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MEN'S FINAL: Roger Federer reacts during the final against Novak Djokovic. (Photo by Clive Brunskill/Getty Images)

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MEN'S FINAL: Novak Djokovic celebrates a point against Roger Federer. (Photo by Maddie Meyer/Getty Images)

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MEN'S FINAL: Roger Federer wipes his face during a break in play. (Photo by Clive Brunskill/Getty Images)

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MEN'S FINAL: Novak Djokovic returns a backhand shot. (Photo by Alex Goodlett/Getty Images)

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MEN'S FINAL: Roger Federer returns a forehand shot. (Photo by Getty Images).

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MEN'S FINAL: Novak Djokovic changes shirts. (Photo by Matthew Stockman/Getty Images)

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MEN'S FINAL: Roger Federer stretches to return a shot back to Novak Djokovic. (Photo by Al Bello/Getty Images)

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MEN'S FINAL: Novak Djokovic stretches after winning the third. (Photo by Clive Brunskill/Getty Images)

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MEN'S FINAL: Novak Djokovic in flight during the final. (Photo by Al Bello/Getty Images)

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MEN'S FINAL: Novak Djokovic celebrates winning a game in the fourth set. (Photo by Clive Brunskill/Getty Images)

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MEN'S FINAL: Novak Djokovic celebrates after defeating Roger Federer during their Men's Singles Final. Djokovic defeated Federer 6-4, 5-7, 6-4, 6-4. (Photo by Clive Brunskill/Getty Images)

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MEN'S FINAL: Novak Djokovic celebrates with his box. (Photo by Matthew Stockman/Getty Images)

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MEN'S FINAL: Novak Djokovic celebrates with the trophy after defeating Roger Federer. (Photo by Clive Brunskill/Getty Images)

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MEN'S FINAL: Novak Djokovic celebrates with the winner's trophy. (Photo by Matthew Stockman/Getty Images)