Searing heat on Monday forced officials at the Australian Open to invoke the extreme heat policy to protect the health of the competitors and played havoc with the day's schedule.

At 1415 (local time), tournament referee Peter Bellenger suspended the start of new matches for an initial period of two hours, although matches in progress continued.

The policy kicked in once the mercury hit 35 degree Celsius and the new Wet Bulb Globe Temperature (WBGT) was 29.

The WBGT is a combination of ambient air temperature, humidity, intensity of solar radiation and wind speed, which together affect the ability of a person to lose heat whether exercising or sitting still.

Women's top seed Serena Williams played through the worst of the heat, beating Eleni Daniilidou in stifling conditions.

"It's definitely very hot out here...it is hard to breathe," she said after her 6-4 6-1 fourth round win. "But it could be worse. In Florida, where I live, when you walk out you lose 20 pounds because of the humidity and heat."

Organisers said Melbourne Park's two main courts were to have their retractable roofs closed once matches already in progress had finished.

But a sudden cooling of the temperature meant they were able to keep them open. Matches on outside courts, however, were still suspended for two hours.

"There is a minimum two-hour break on outdoor courts when the policy kicks in. This is because we can't have players phoning up every 10 minutes asking 'has the temperature dropped yet? Are we able to play yet?'," tournament spokesman John Lindsay said.

"They know they can go away for at least two hours before anything happens."

"On the two main courts, however, we can control the conditions. Players scheduled to play on those courts know they will be playing, whatever happens, so we are better able to make last-minute decisions."