* Play halted as temperatures reach 43.4 degrees Celsius
* Play continued on show courts when roofs closed
* Lightning and rain stops play (adds rain stoppage in third paragraph)
MELBOURNE, Jan 16 (Reuters) - Organisers lost more than four hours of play on the outer courts when a third day of high temperatures at the Australian Open forced them to enact their "Extreme Heat Policy" on Thursday.
The policy was put into force at 1.50 pm local time (0250 GMT) as the mercury headed towards a peak of 43.4 degrees Celsius (110 Fahrenheit) and no play was possible until 6 pm.
In a bizarre turn of events, less than two hours after the resumption, the players were forced off the courts again by lightning and rain.
Play continued on the Rod Laver Arena and Hisense Arena through both stoppages after the retractable roofs over the main showcourts were closed.
Organisers had been slammed for forcing players to play on in searing temperatures on Tuesday and Wednesday and the decision to stop play was largely welcomed.
"I think it's everybody saying that sometimes it's even too hot," said Agnieszka Radwanska, who beat Olga Govortsova 6-0 7-5 under the closed roof on Hisense Arena.
"Some of the girls can't even talk after the match or practise. You can see who played a match, you know. Just so red.
"Today was really, really hard. Even indoors was ridiculous."
Under a change to the rules for this year, the decision on whether to stop matches at the tournament is now at the discretion of tournament referee Wayne McKewen.
Rather than use the raw Celsius readings to assess the heat, organisers prefer to use the Wet Bulb Global Temperature composite, which also gauges humidity and wind to identify the perceived conditions.
"Today the key parameters to determine whether play should be suspended were reached," said the tournament's chief medical officer Dr Tim Wood.
"The ongoing forecast was for even warmer conditions and therefore the decision was made to suspend play on the outside courts until weather conditions improved."
With players having to finish the ongoing set before play ceased or the roofs were closed, however, Maria Sharapova's match on Rod Laver Arena continued in the full glare of the sun for 50 minutes after the policy was enacted.
The third seed eventually finished off Italian Karin Knapp 6-3 4-6 10-8 to reach the third round.
"There is no way getting around the fact that the conditions were extremely difficult, and have been for the last few days," Sharapova told reporters.
"It's a tough call. I mean, I think the question I have is no one really knows what the limit is.
"Not the players (nor) the trainers themselves when you ask them when will the roof be closed."
American Varvara Lepchenko clearly struggled in the heat in the first match on court eight, which she lost 4-6 6-0 6-1 to Romanian Simona Halep.
"I think they definitely should have not started the matches at first place," Lepchenko, who had to be iced down during one changeover, said.
"I think they should have started the matches after the temperature cooled down a little bit because this is just too much.
"Obviously it is very dangerous if someone has conditions with their heart or anything like that or just being in this temperature it's almost like going to (the) sauna."
On Tuesday, when temperatures peaked at 42.2 degrees, Canadian Frank Dancevic passed out during his first round match and accused organisers of forcing players to play in "inhumane" conditions.
Ivan Dodig became the 10th player to retire in the first three days of the tournament on Wednesday and said he feared for his life after being rendered immobile by the heat on the exposed outer courts.
Twelve doubles matches scheduled for day four were cancelled and will be played at a future date.
The hot weather is forecast to continue through Friday before a dramatic drop in temperatures at the weekend. (Additional reporting by Daniel Hughes; Editing by John O'Brien)