Concerned organisers were forced to switch the floodlights on early at the China Open today when thick smog drifted onto the showcase courts.
Spotlights illuminated the gloomy conditions at the Olympic tennis centre in Beijing just before 4pm local time.
Air quality monitors at the US China Embassy recorded hazardous levels in the Chinese capital for a second day running with light being badly affected.
The official China Environmental Monitoring Center, however, said the air was "slightly polluted"
"You don't really think about it. You're just so focused that you don't really feel it," Caroline Wozniacki said after her 6-4- 6-2 win against Sara Errani last night.
The 20-year-old Dane will be crowned world number one if she beats Czech Petra Kvitova this evening - dethroning American Serena Williams.
WTA and ATP tour officials said they will investigate the effects of the smog on both players health and the image of the combined tournament.