Up first on centre court was a hit out between former world number one, Maria Sharapova, and 14th seed Anastasija Sevastova.
Novak Djokovic on Thursday described the conditions as "brutal", complaining it was hard to breathe and they were "right on the limit".
Still looking for a first major, top-ranked Simona Halep held off 2014 Wimbledon finalist Eugenie Bouchard 6-2, 6-2.
Muguruza said her feet were suffering in the heat as temperatures soared above 39.
He defeats Monfils in four sets, winning 4-6 6-3 6-1 6-3.
"He wasn't at his best at the end of the second and entire third set".
Later the young Aussie told reporters what he was telling Smith during the changeovers of the third set.
In the second stanza, the six-time Australian Open champion continued to dominate, although there was a point when those in attendance feared an injury may be about to end Djokovic's chances.
Sandgren packs Wawrinka off Struggling Swiss former victor Stan Wawrinka was knocked out by American Tennys Sandgren in the second round. The Spaniard beat Tim Smyczek of the USA 6-4, 6-2, 7-6 (7-2).
The six-time champion looked like a man who had been sidelined for six months during the first set, making uncharacteristic errors at bad times and finding his remodelled serve given a pounding. Djokovic broke for 5-3 before sealing the set on serve.
"It was obvious we both suffered on the court today, " Djokovic said.
The 12-time Grand Slam champion has a day to recover for his third round encounter with Spanish 21st seed Albert Ramos-Vinolas.
"They're at very high risk of overheating and that is potentially lethal".
Especially as they were literally dropping like, er, flies.
'We work closely with our medical team to ensure players are educated on best practice in terms of preparing for the conditions, and as we know many of them come to Australia early so they can acclimatise.
"I have Focus on my phone and watch it every time I have a flight. It was too hot on court today", she smiled, adding that it hadn't been a problem for herself, the world humber 88. Multiple tweets from tennis fans translated Tsonga's words as: "Come down here I'm going to kick your a**", claiming it was directed at someone in Kyrgios' box. "It was about just hanging in there", he said. "I just wanted to play the match and to win it", the 26-year-old Romanian said.
"The players just need to toughen up and get on with it".
The temperature climbed to 69C in the middle of Rod Laver Arena and there were calls for the match to be stopped or at least for organisers to close the roof to soften what Djokovic called the "brutal" conditions.
"That was really ugly", she said of the weather.
"Billie Jean invited me to the opening in LA".
"I would not like to see here retirements".
"Recovery is not something I worry about with him", Agassi said while promoting Lavazza coffee as its global tennis ambassador. She appeared to be overcome by the heat. "I think I had a small heat stroke for 40 minutes". I don't have anything against French (smiling). I didn't really say anything.
-Angelique Kerber on turning 30. "But I also think that there should be some kind of rational conversation about rules that are maybe imposed or certain things that are concerning players' well-being".
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