A spectator at the AUS Open is seen covered in towels to ward off the Melbourne heat. Photo: Lukas Coch/BackpagePix

MELBOURNE – Caroline Garcia said her feet were on fire and Dominic Thiem couldn't wait to get in an ice bath as Melbourne's searing summer heat took its toll at the Australian Open 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.”

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.

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