Spectators in Beijing expressed concern for the players' health as they donned protective masks to watch the matches.
"There is definitely danger to the athletes," said spectator Ren Yumei, 39, outside the National Tennis Center.
"I think either the sponsors or government should have solved this in advance, but neither of them did."
Some were unperturbed by the smog however. "It's not like this air pollution never existed in Beijing before," said a woman surnamed Du.
"When an international game like this is held in the city, I am still keen to watch it."
On Wednesday, China's National Meteorological Center (NMC) issued a yellow alert as smog blanketed parts of northern China, cutting off visibility in Beijing and nearby Tianjin to just over half a mile.
The NMC warned people in the affected to "reduce outdoor activities"... like tennis.
On Monday, French player Jo-Wilfried Tsonga complained of dizziness and had to receive medical treatment during his defeat by Austrian Andreas Haider-Maurer.
While Tsonga refused to speculate on whether the smog was the cause of his problems, world No. 42 Martin Klizan was less circumspect.
In a Facebook message, the Slovakian said that the pollution had caused "uncontrollable" coughing and vowed not to play in Beijing again unless conditions improve. Though Klizan later deleted the post, screenshots remain online and were widely shared on social media.
World No. 1 Novak Djokovic seemed unfazed by the pollution -- though he has described the air quality as "not ideal" in previous years -- as the Serbian, who has won the tournament five times since 2009, smashed Italian Simone Bolelli to progress to the next stage on Tuesday.
Representatives for the ATP, which organizes the tour, and the China Open, did not respond to requests for comment.
Pollution has long been a problem in Beijing
-- the city was deemed almost "uninhabitable for human beings" by a study carried out last year by the Shanghai Academy of Social Sciences.
Residents got a rare break this summer, as the government shut down hundreds of factories and ordered half the capital's five million registered cars off the roads to ensure blue skies for a parade to mark the 70th anniversary of Japan's defeat in World War II.