Gael Monfils said he was "dying" on court, Caroline Garcia's feet were burning up and Dominic Thiem couldn't wait to get in an ice bath as Melbourne's searing summer heat played havoc at the Australian Open on Thursday.

The opening Grand Slam of the year routinely challenges players and fans with its fickle weather - it can be 20 Celsius one day and 40 Celsius the next.

This week the heat has been gradually building and it touched 40 Celsius (104 Fahrenheit), with even worse forecast for Friday.

Some players cope better than others in the furnace-like conditions, with third seed Garbine Muguruza one of those who suffered, crashing out in straight sets.

"I think the surface of the court, I don't know how much heat, it's terrible, very, very hot, and it's easy to get blisters and red," she said.

"I had a more tougher match under the heat in previous years at the Australian Open, but today was -- it was hot, but I don't think was the hottest day."

There were concerns for Monfils' health in his mid-afternoon match against Novak Djokovic, with the Frenchman, known as one of the fittest players on tour, looking dazed and confused in the second set.

He eventually got medical assistance and could be heard pleading for more time between points in the stifling heat

"I got super dizzy, I think I had a small heatstroke for 40 minutes," he said after losing in four sets. "I'm telling you, I was dying on the court for 40 minutes."

Djokovic described the conditions as "brutal".

"It was obvious we both suffered on the court today. They were really tough conditions. It was brutal for the first hour and a half,” said the Serb.

French eighth seed Garcia spent three gruelling sets in the sun against Marketa Vondrousova before limping to a 6-7 (3/7), 6-2, 8-6 victory.

"It's definitely hot," she said afterwards. "My feet are burning. But we know it's like this in Australia -- the next day it can be freezing."

The conditions present the ultimate test of fitness and stamina, with organisers only activating the extreme heat policy when the temperature exceeds 40 Celsius and the wet bulb globe temperature index hits 32.5 Celsius.

Tournament organisers said they had been "close" to calling a halt to play and shutting the main stadiums roofs.

Maria Sharapova was untroubled by the heat, although she was on court in the morning and avoided the worst of it.

"I love the summertime," she said breezily after dispatching 14th seed Anastasija Sevastova in straight sets.

Players are urged to keep themselves hydrated and use ice wrapped in towels to cool off at changeovers in the draining weather.

Fifth seed Thiem took the advice to heart as he struggled through a energy-sapping five-setter against Denis Kudla.

"I'm off to have an ice bath," was all he managed to say after the marathon match.

A cooler change is expected to blow through Melbourne from Saturday

Source: AFP