A blanket of smoke from massive wildfires in Australia hangs like a blanket over the city of Melbourne. The smog there is so thick that some of the world's top athletes have raised alarms about player safety at the Australian Open tennis tournament, slated to kick off next week.

The air quality in Melbourne on Wednesday was forecast to be "very poor to hazardous," according to the Environment Protection Authority in Victoria state.

The hazardous breathing conditions prompted Australian Open officials to suspend practice sessions Tuesday. But qualifying matches went on as scheduled, and one of the players later said it was "not fair" that they were asked to compete.

That player, Slovenia's Dalila Jakupovic, was leading 6-4, 5-6 in her match against Switzerland's Stefanie Voegele when she was overtaken by a fit of coughing and dropped to her knees. Her breathing difficulties forced her to forfeit, handing the victory to Voegele.

"I was really scared that I would collapse," Jakupovic told the Australian Associated Press. She said she does not suffer from asthma or have a history of respiratory issues. "I just couldn't breathe anymore and I just fell to the floor."

Jakupovic wasn't the only one with breathing difficulty.

The AP also reports Eugenie Bouchard received medical attention during her qualifying match, which she eventually won.

Over at the Kooyong Classic, Russia's Maria Sharapova was down a set in her exhibition match against Germany's Laura Siegemund. Tied at 5-5 in the second set, officials moved to stop play as both players struggled with the heat and smoke.

"It was the right call from the officials," Sharapova told SBS Sports.

The official account for the Australian Open tweeted Monday that event staff members were monitoring onsite data and working with medical personnel and scientists to make sure that it is safe for players to take to the court. "As always the health and safety of our players, our staff and our fans is our priority."

Bushfires in Australia have claimed the lives of more than two dozen people, destroyed nearly 3,000 homes and charred thousands of acres since they began in September. University of Sydney ecologist Chris Dickman has estimated that more than a billion animals may have been killed in the fires.

Australia is no stranger to wildfires, but this year's fire season is historic. Two massive fires recently combined to form a "megafire" more than three times as large as any known fire in California, as NPR has reported.

The bushfires have also produced a plume of smoke the size of the continental United States and pumped hundreds of millions of tons of carbon dioxide into the atmosphere.

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