"But there is a limit of being fit to play and being, I think, in danger to your health".
Top-seeded Simona Halep served for the match four times before prevailing 4-6, 6-4, 15-13 over American Lauren Davis in a marathon match at Rod Laver Arena.
The third set was a one-sided affair, as Djokovic took full advantage of his opponent's significant drop-off.
Alexander Zverev advanced in four sets over Peter Gojowczyk while Dominic Thiem came back from two sets down to win in five against Denis Kudla 6-7 (6), 3-6, 6-3, 6-2, 6-3.
Players have been using ice towels to cool down during the change of ends.
And Friday promises to be worse. It was great for me to actually get into that long exchanges and rallies, to get some rhythm on the court after the Gael Monfils match, which was really up and down, unusual conditions of play. "At times we were both just trying to get a little bit of extra breath, a few seconds more, so we could recover".
Many expected Kerber's clash with Sharapova to be another thriller, although Kerber, the champion here in 2016, made light work of the Russian, who was the victor in Melbourne back in 2008.
"But what I see is a strong Novak who knew how to manage the weather and find solutions in the toughest second round of the tournament, in my view".
Tournament organisers said they had been "close" to calling a halt to play and shutting the main stadium's roofs.
"For sure we took a risk", said Monfils. "It's a bit risky". But I do believe, if you break it down to layman's terms, an official can stop play when it gets really hot. "I'm telling, I was dying on the court for 40 minutes", he said. I think I had a small heat stroke for 40 minutes.
"I may have thrived in those conditions", he said.
'I'm really happy that we got a chance, all of us players, to speak together in one place for an hour, ' he said. "So I don't want to sound ungrateful".
In a battle of the old stagers, Czech 19th-seed Tomas Berdych had too much firepower for Spain's Guillermo Garcia-Lopez and posted a 6-3, 2-6, 6-2, 6-3 victory.
Friday's torturously hot conditions at the Australian Open in Melbourne, have raised serious questions about player safety and whether the event should be moved to a different time of year, outside of the Aussie summer season.
Monfils saved two more match points, the second with a superb crosscourt forehand, but Djokovic converted at the fourth attempt with a simple volley.
After defeating Jan-Lennard Struff in straight sets, Roger Federer is expecting a tough third-round clash against Richard Gasquet. Among the men, seventh-seeded David Goffin was shocked by Frenchman Julien Benneteau 6-1, 6-7 (5-7), 1-6, 6-7 (4-7).
That left unseeded Maria Sharapova, No. 21 Angelique Kerber and French Open champion and No. 7 Jelena Ostapenko as the only Grand Slam winners in the womens draw.
In mixed doubles play Canada's Gabriela Dabrowski and Mate Pavic of Croatia are moving on.
Argentine Nicholas Kicker, who defeated Australia's Jordan Thompson in the first round, pushed past Slovak Lukas Lacko 6-2, 7-5, 1-6, 7-5.