Extreme heat halts play at Australian Open
Maria Sharapova wears an ice towel during her Australian Open match against Karin Knapp in Melbourne on January 16, 2014
Players were to complete their current sets before a suspension until at least 5:00 pm (0600 GMT) on outside courts, they said, while the roofs were closed on the centre and second courts.
"The Australian Open extreme heat policy has been implemented," said a statement.
"The roof on Rod Laver Arena and Hisense Arena will be closed at the end of the set. Play will be suspended at the end of the set on all outside courts until further notice."
The move, after two days of soaring temperatures and complaints from players, will bring severe disruption to Thursday's packed schedule of 32 singles matches plus doubles.
Temperatures were forecast to reach 44 Celsius on Thursday, the hottest this week, and remain high on Friday before cooling sharply on Saturday.
The tournament's opening days have been dominated by problems associated with the heat, with players falling dizzy, fainting and vomiting, and warnings of serious health problems.
"Doing physical exercise in this heat is just unbelievable," said France's Alize Cornet. "Even for the fans, sitting in the sun must be terrible."
America's Varvara Lepchenko was the latest victim on Thursday when she had her pulse and blood-pressure checked and was rubbed down with ice by staff during a medical time-out.
With the court too hot to lie on, she was treated while reclining on a row of chairs. Outside the venue, tramlines have buckled in the heat and a state-wide fire ban is in effect.
Temperatures had already reached 38 Celsius as play started at 11:00 am, with players shrouding themselves in ice packs and gulping drinks at the changeovers.
Canada's Frank Dancevic, who hallucinated that he was seeing Snoopy and blacked out during Tuesday's loss to Benoit Paire, has called conditions "inhumane" and "hazardous".
Croatia's Ivan Dodig said he feared he might die after pulling out of his match with Damir Dzumhur with severe cramp on Wednesday. And Chinese player Peng Shuai cramped and vomited in the severe heat.
Wimbledon champion Andy Murray has said organisers were risking a serious incident by letting play continue, warning of the chance of a player suffering a heart attack.
French seed Richard Gasquet agreed with Murray, saying: "That's true, it's dangerous. It's not right to play under such conditions."
Thursday's forecast high is just short of Melbourne's January record of 45.6 Celsius, which came during the notorious Black Friday bushfires of 1936.
The Australian Open, played at the height of the Melbourne summer, is notorious for its hairdryer heat.
In 2009, the hottest edition on record with an average daily temperature of 34.7 Celsius, reigning champion Novak Djokovic pulled out of his quarter-final with Andy Roddick, citing heat exhaustion.
The current heatwave is one of Melbourne's hottest and is comparable to 2009, when severe bushfires in the surrounding state devoured entire towns, killing 173 people.
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