Tournament co-director Charles Hsiung said there were no specific measures, such as breathing apparatus, in place for the players despite pollution which is often so high that authorities warn against outdoor exertion.

Smog reached "hazardous" levels at this year's tournament before clearing during the final days, and one player complained that he vomited after a match because of the haze.

"Compared to many days in the previous years you can see a remarkable improvement in terms of our weather conditions," Hsiung told a press conference, in response to a question from AFP.

"Is there a rule that says (you can play) to a certain level of pollution? Well there should exist some sort of a rule but definitely we did not hit that rule in any way whatsoever."

When pressed about when play might be halted, Hsiung said: "If the sight of the balls, to the extent that you can't see the ball, obviously you can't play. But we are far from that level of pollution."

Pollution is a chronic problem in Beijing. In 2010 Novak Djokovic said players should have oxygen tanks on court, and two years ago Sweden's Robert Lindstedt called it "the city that cuts off days from your life".

Fans in facemasks were again a common sight this year, and world number 42 Martin Klizan said the "extreme smog" made him "cough uncontrollably after every point and I had to vomit after the match".

Jo-Wilfried Tsonga was one of three players to fall victim to dizzy spells in the first round, when the haze was high, although one of them, Eugenie Bouchard, was concussed in recent weeks, andPetra Kvitova blamed the glandular fever she suffered earlier this year.

"I don't know. You know, nothing in me can calculate if it's enough oxygen for me or not. I just play tennis," said Tsonga, when asked if he thought it was the pollution that had made him feel faint.

Klizan later deleted his comments, made on Facebook. Most competitors have politely played down concerns over air quality or say that, after several years visiting China, they are used to it.

Hsiung did not detail any special medical facilities for the players but said there were spaces indoors where they can go when the pollution is high.

"We have a lot of indoor facilities which include (a new) gym, so players can really enjoy themselves and do stretching etc indoors, should the weather get so bad," he said.

"But honestly it's not really a lot of pollution this year as compared to previous years."

China hosts a rising number of top sports events, especially tennis, with eight Women's Tennis Assocation and three men's Association of Tennis Professionals tournaments this year.