"It's really tough on your feet to play in these conditions", she said.
Aussie Ash Barty was far luckier, playing in the evening against Italian Camila Giorgi.
Lower seeds upholding their standings on Thursday were No. 17 Madison Keys in blistering form, stomping Russian Ekaterina Alexandrova 6-0, 6-1; No. 18 Ashleigh Barty defeating Camila Giorgi 5-7, 6-4, 6-1; No. 20 Barbora Strycova beating Lara Arruaberrena 6-3, 6-4; and the No. 21 Kerber who rolled past Donna Vekic 6-4, 6-1. "It had to come eventually, this injury has been with me for a while, it escalated a year ago and I had to take some time off".
The hottest topic of the Australian Open is once again the heat after a day in which Melbourne sweltered and players just about got roasted on court. "At times I mind that, I don't like that".
'I think it was a bit tougher for him [the umpire] today.
For Barty, the match followed a similar script to her first-round win over Aryna Sabalenka as she came from behind against a big-hitting opponent.
Austria's fifth seed Dominic Thiem fought back from two sets down to overcome American qualifier Denis Kudla and fierce heat to reach the third round.
Barty will face Japanese teenager Naomi Osaka in the third round on Saturday.
Monfils trained in the heat of Miami in the off-season - a base in the past for three-time grand slam champion Andy Murray - but said it was no match for what he encountered on Rod Laver Arena. "I follow my heart, a path lights up and something tells me to put my energy there and I trust it".
Monfils broke Djokovic's opening two service games to get off to a flyer, only to give back a break with a double-fault on break point.
So the 35 degree Celsius (95F) heat may have quickly forced a rethink as Sharapova burst out of the blocks in a hail of winners to roar through the first set in 23 minutes.
Monfils struggled to cope.
"For sure, we took risk".
After his 4-6, 6-3, 6-1, 6-3 victory Djokovic described the conditions as "brutal" and said they were "right at the limit" in terms of being a danger to health.
"There is a rule [that we use] about index, combination between temperature and humidity", he said.
But she later said it was "dangerous" to play in that kind of heat.
Del Potro beat Karen Khachanov in four sets before he too complained about the weather. What is most important for us is our health and what happens after our career, after you're 30, 35.
"But I survived. I will play after tomorrow I hope in different conditions".
On a day when shade was a valuable commodity in the searing heat at Melbourne Park, Roger Federer played it cool at the Australian Open.
Meanwhile, Czech Republic's Karolina Pliskova beat Brazil's Beatriz Haddad Maia 6-1, 6-1 in just 44 minutes to set up the third round clash with the her compatriot Lucie Safarova, who defeated the Romanian Sorana Cirstea 6-2, 6-4 in the second round.
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