The American, trying to win the title for a sixth time, dispatched Vesna Dolonc of Serbia 6-1 6-2 in just over an hour on Rod Laver Arena before scurrying back to the cool sanctuary of the locker room.
"I kept waking up in the middle of the night last night, just paranoid," Williams said of her disrupted sleep.
"I just wanted to stay hydrated. The last thing I want to do is to cramp in this weather. It can happen so easy. I was just drinking a tremendous amount of water."
With temperatures topping 40 degrees Celsius (104 Fahrenheit) for a second successive day, Williams was never troubled by the world number 104 and cruised to victory.
On Tuesday, defending champion Victoria Azarenka said being on court was like "dancing in a frying pan", while Canada's Frank Dancevic slammed organisers for forcing players to compete in inhumane conditions after losing his first round match.
Williams said she tried not to change her game-plan because of the conditions and said the weather was just about bearable.
"I think I probably feel it more in my head, not necessarily in my chest," she said of the heat. "But so far I have been OK.
"I have been training for a long, long time in the heat in Florida and I have been coming to Melbourne for years and years.
"So you just have to be ready for those hot conditions and just try to get through it.
"I think the conditions today in my match was OK. I wasn't here yesterday. I heard they were a little bit extreme (but) I didn't really even go outside yesterday. For my match today, it was OK to play."
The victory was her 60th in the main draw at the tournament, which tied her with Australian Margaret Court for the most wins in the event's Open era.
Williams, who now plays either 31st seed Daniela Hantuchova of Slovakia or Czech Karolina Pliskova, said the heat can affect players differently on any given day.
"That definitely can be true, she said. "I'm not sure.
Sometimes my body just says 'no' to hot weather. Sometimes I have no reaction.
"I'm just happy that I was able to play today before it got super hot and it felt really good."