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The National Tennis Stadium in Beijing, China is seen enveloped in pollution haze during the second-round women's singles match between Ana Ivanovic of Serbia and Venus Williams of the United States at the China Open tennis tournament on October 6, 2015. Andy Wong/AP
Spectators wearing masks watch the match between Ivanovic and Williams on October 6. Andy Wong/AP
Williams, right, serves against Ivanovic -- the American lost 7-6 (7-3) 6-2 as the Serbian kept alive her hopes of qualifying for the season-ending WTA Finals. Andy Wong/AP
A man shields his nose from the smog during the match between Belinda Bencic of Switzerland and Madison Brengle of the United States at the National Tennis Stadium on October 5, 2015. Emmanuel Wong/Getty Images AsiaPac/Getty Images
Mask-wearing spectators watch the first round women's singles match between Belinda Bencic of Switzerland and Madison Brengle of the United States on October 5. Andy Wong/AP
Jo-Wilfried Tsonga of France touches his chest after receiving a medical time out during a match on October 5. Tsonga complained of dizziness and shortness of breath but would not comment on whether he thought the smog was the cause of his problems. Andy Wong/AP
Tennis fans protecting themselves from pollutants walk past the National Tennis Stadium during the China Open tennis tournament in haze-hit Beijing on October 5. Andy Wong/AP
