As Roger Federer reached a landmark 300th Grand Slam win, Djokovic still looked like the man to beat as he weathered a strong challenge from Andreas Seppi to reach the first weekend without dropping a set.
The top seed and defending champion saved two third-set points against Seppi before reeling off the next four points to clinch it 6-1, 7-5, 7-6 (8/6), his 33rd straight victory over Italian opposition.
Federer earlier went through 6-4, 3-6, 6-1, 6-4 against Grigor "Baby Fed" Dimitrov, a man whose game is so similar to his own that he admitted it was like playing a mirror.
It wasn't all plain-sailing for Federer, but with the win he became the first man to reach 300 Grand Slam victories and lies six away from Martina Navratilova's record of 306.
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"It's very exciting, I must tell you," said the Swiss, who also became the oldest man to reach the round of 16 since Andre Agassi in 2005.
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Sharapova left the court after she lost the second-set
tiebreak but after she came back refreshed and revitalised, she raced to victory to set up a clash with Switzerland's Belinda Bencic.
Williams had no such problems as she crushed overawed Russian teenager Daria Kasatkina 6-1, 6-1 in 44 minutes, the shortest match of the tournament so far.
"I definitely think I played better today. Everything I've been trying to work on was kind of clicking today," said the buoyant world number one, who is seeking a record- equalling 22nd Grand Slam title.
Belgium's David Goffin awaits Federer in round four, after he beat Dominic Thiem 6-1, 3-6, 7-6 (7/2), 7-5 in a match which included a generous act of sportsmanship from the Austrian.
Thiem, leading in the second set, told the umpire to give a point to Goffin rather than replay it after a Hawk-Eye challenge found a shot from the Belgian had landed in.
Elsewhere, fourth seed Agnieszka Radwanska reeled off nine straight games to floor Monica Puig 6-4, 6-0, despite continuing pain from a left-leg injury which stopped her competing in Sydney last week.
In other matches, Japan's Kei Nishikori beat Guillermo Garcia-Lopez 7-5, 2-6, 6-3, 6-4 and Jo-Wilfried Tsonga ousted fellow Frenchman Pierre-Hugues Herbert 6-4, 7-6 (9/7), 7-6 (7/4).