Williams defeated No. 31-seeded Daniela Hantuchova 6-3, 6-3. Hantuchova had six break chances but only converted one while Williams had a pair of breaks in each set.
It was the 61st match victory for Williams at the Australian Open, topping the record set by Margaret Court. Williams hasn't lost a match since last August, when Victoria Azarenka beat her in the final at Cincinnati.
Williams was on the court 1 hour, 20 minutes in one of the early matches Friday.
Even then, temperatures were 103 degrees Fahrenheit and eventually reached 109 degrees. The heat wave broke later in the day and temperatures are predicted to be in the mid 70s for the weekend but rain, which caused two suspensions of play Friday, is forecast for Saturday and Sunday.
Li Na, seeded fourth and the 2013 Australian Open runner-up, saved a match point before taking a 1-6, 7-6 (7-2), 6-3 win over 26th-seeded Lucie Safarova.
Safarova won more than two-thirds of the points of the first set and was up 5-3 in the second. Li fought back to 5-5 but Safarova had a potential match-winning point go just long and Li took advantage of the opening to escape with a win. Li advanced despite losing her serve six times and committing 50 unforced errors.
The only other women's match to go three sets Friday was the last one of the day when 14th-seeded Ana Ivanovic beat No. 17 Samantha Stosur 6-7 (8-10), 6-4, 6-2 before a partisan crowd supporting Stosur, who is from Brisbane.
Perth native Casey Dellacqua, however, kept Australian hopes alive with a 6-2 6-4 win over Zheng Jie. Dellacqua is the only unseeded woman to make it to the fourth round in the top half of the draw.
No. 9-seeded Angelique Kerber beat Alison Riske 6-3, 6-4 win seven breaks and No. 22 Ekaterina Makarova stopped Monica Niculescu 6-4, 6-4. Also, 28th-seeded Flavia Pennetta was a 6-1, 7-5 winner over Mona Barthel and Eugenie Bouchard, seeded 30th, beat Laura Davis 6-2, 6-2.
Williams will take on Ivanovic in the highlight match of the fourth round. Other pairings have Dellacqua playing Bouchard, Li going against Makarova and Kerber playing Pennetta.