The temperature hit 39 Celsius (102 Fahrenheit) during Williams' match and was forecast to climb to 44 C (111 F) later Friday, with match suspensions a possibility on outer courts for the second consecutive day.
"It was a tough match ... it's definitely hot, but you have to be ready to play," she said. "And then you have to prepare yourself mentally, too."
Williams has won 24 matches dating back to August, carrying momentum from her dominating 2013 into a new season. She won 78 of her 82 matches and won 11 titles last year, and seems to be getting better with age.
"I feel like, in life 32 is young. In sports it's old," she said. "But for whatever reason, I feel like I just never was really able to reach my full potential, and I feel like recently I just have been able to do a little better. I just keep trying to improve on everything."
Williams was entered in the doubles with her sister but withdrew after her singles match Friday, citing a leg left injury for Venus, which saved her from another match in the blazing heat in the afternoon.
"I love playing doubles, love playing with Venus, but sometimes it doesn't work out," she said. "I guess it's a relief, but like I say, we love to play doubles and we love to be out there ... so I'll definitely miss it."
Rod Laver Arena was about three-quarters full for the match, though it was a quiet crowd people seemed to be expending more energy fanning themselves with programs and newspapers than applauding points on the court.
No. 9 Angelique Kerber advanced earlier with a 6-3, 6-4 win over American Alison Riske, and said her main aim had been to "get off the court before it became really hot." Kerber will next play No. 28 Flavia Pennetta, who advanced 6-1, 7-5 over Mona Barthel of Germany.
No. 22 Ekaterina Makarova advanced with a 6-4, 6-4 win over Monica Niculescu and No. 30 Eugenie Bouchard beat American Lauren Davis 6-2, 6-2.
In an early men's result, Florian Mayer beat No. 20 Jerzy Janowicz 7-5, 6-2, 6-2.
Matches on outer courts were suspended for four hours on Thursday afternoon when the Extreme Heat Policy was invoked for the first time in the tournament. Lightning and rain caused further delays later in the evening. Maria Sharapova advanced after playing 3 ½ hours in the blazing sun, before the roof was closed on Rod Laver Arena.
Two-time defending champion Victoria Azarenka won her night match indoors. The loaded top half of the men's draw lost a contender when No. 5 Juan Martin del Petro lost a second-round match that ended at 1:20 a.m. Friday. Rafael Nadal, Andy Murray and Roger Federer all progressed earlier on day four.
R Serena Williams' 61st career victory at the Australian Open is the most of any woman.
• Williams wins 6-3 6-3 over Daniela Hantuchova in 102 degree weather.