Li Na, the 2011 French Open champion and a two-time finalist at Melbourne Park, had a 6-0, 7-6 (5) win over 16-year-old Belinda Bencic and will next play No. 26 Lucie Safarova in the third round.
No. 9 Angelique Kerber advanced and will next meet American Alison Riske, who trounced Yanina Wickmayer 6-1, 6-1. Australian wild-card entry Casey Dellacqua upset No. 18 Kirsten Flipkens 6-3, 6-0.
No. 17 Sam Stosur is through to the third round of her home Grand Slam for the first time in three years after a 6-2, 6-0 win over Tsvetana Pironkova. No. 30 Eugenie Bouchard of Canada beat Virginie Razzano 6-2, 7-6 (10) and Zheng Jie defeated American Madison Keys 7-6 (5), 1-6, 7-5.
Stosur will face former French Open champion Ana Ivanovic in the third round after the Serbian player beat Annika Beck of Germany 6-1, 6-2. The Stosur-Ivanovic winner could face Williams in the round of 16.
The tournament's heat rule went into effect in the first match on Margaret Court Arena, giving No. 15-seeded Sabine Lisicki and Monica Niculescu a 10-minute break after the second set. Niculescu won 2-6, 6-2, 6-2 and will next play No. 22 Ekaterina Makarova, who followed up her win over Venus Williams with a 6-2, 7-5 victory over American Irina Falconi.
Djokovic had a brief scare while serving at 3-0 in the first set when he turned over on his left ankle, tumbling to the court.
"It was just I slipped a little bit," Djokovic said. "It was an instant discomfort, but it was OK after."
No. 3 David Ferrer beat Adrian Mannarino of France 7-6 (2), 5-7, 6-0, 6-3 to advance along with No. 7 Tomas Berdych and No. 9 Richard Gasquet, No. 17 Tommy Robredo, No. 20 Jerzy Janowicz and No. 29 Jeremy Chardy.
In late-finishing matches, eighth-seeded Stanislas Wawrinka beat Alejandro Falla of Colombia 6-3, 6-3, 6-7 (4), 6-4, No. 15 Fabio Fognini beat Jarkko Nieminen 7-5, 6-4, 3-6, 6-2 and No. 28 Vasek Pospisil of Canada overcame a back injury with the help of painkillers during the match to beat Australia's Matt Ebden 3-6, 7-6 (6), 7-6 (9), 6-1.
Florian Mayer beat No. 14 Mikhail Youzhny, Uzbekistan's Denis Istomin defeated No. 30 Dmitry Tursunov and Bosnian qualifier Damir Dzumhur advanced when No. 32-seeded Ivan Dodig retired with cramps while leading in the fourth set.
Sam Querrey fired 19 aces, saying he had "one of the best service days of my life" in a 6-2, 6-3, 6-4 upset win over No. 23 Ernests Gulbis.