The Australian Open tennis Grand Slam began on schedule Monday after air pollution eased following a build-up plagued by choking haze from widespread bushfires.

Japan's Naomi Osaka opened her title defence against Czech Marie Bouzkova on Rod Laver Arena, the centre court, under cloudy skies in Melbourne with rain forecast.

Tournament officials are closely monitoring air quality and will halt play and close the three main stadiums' roofs if the particulate matter rating (PM2.5) -- the solid and liquid particles suspended in the air -- hits 200.

Several players had coughing and breathing problems during qualifying last week, when haze blew over Melbourne from the deadly bushfires that have swept Australia.

Slovenia's Dalila Jakupovic was forced to retire from her qualifier after a distressing coughing fit, while Britain's Liam Broady claimed "multiple" players needed asthma medication.

"After I practised indoors I felt really bad with my lungs," said Greece's Stefanos Tsitsipas, the world number six.

"I was coughing a lot had troubles breathing for a couple of hours," he added.

Organisers had repeatedly promised to start the season's first Major on time. Air quality improved markedly over the weekend and was rated as 'good' by Victoria's state environmental agency on Monday.