It was a blip as the Russian played clinical tennis to race through the next five games.
It was only when Sharapova first served for the match at 5-1 that Williams provided substantial resistance, breaking serve for the first time.
That prolonged the match for two more games. But it at least meant Williams avoided equalling the worst scoreline of her 264-match grand slam career, her 6-1 6-1 loss to Martina Hingis in the semi-finals of the 2001 Australian Open.
Sharapova will meet unseeded Belgian Kirsten Flipkens in the fourth round, after Flipkens beat Russia's Valeria Savinykh yesterday.
Earlier, Novak Djokovic didn't get everything his own way against Radek Stepanek despite a 6-4, 6-3, 7-5 result that extended his winning streak to 17 matches at the Australian Open.
Djokovic broke the 34-year-old, No 34-ranked Stepanek late in each set, but was troubled at times against a wily veteran who mixed up the tempo with a lot of serve-and-volley and some unorthodox shot-making.
"Absolutely it was great. Great match and great fun," Djokovic said. "It's always tricky to play Radek. He's a talented player. Skilful player."
Told the statistics showed Stepanek won 36 of the 67 points he played at the net, Djokovic wasn't surprised at all. "He's skilful on the net and he was not giving me a lot of rhythm, he was changing up the pace on the ball," Djokovic said.
"Nowadays everything is based on the baseline. It's nice to see somebody coming to the net."
Djokovic, who won the title at Melbourne Park in 2008, 2011 and last year, is hoping to be the first man in the Open era to win three consecutive Australian titles. Tomorrow, he will play Swiss Stanislas Wawrinka, who beat Sam Querrey (US) 7-6(6) 7-5 6-4.
Angelique Kerber survived some nervous moments before fending off American teenager Madison Keys 6-2, 7-5 in their third-round match, then celebrated her 25th birthday.
Local organisers brought Kerber a cake on to the court immediately after the match and the crowd at Rod Laver Arena sang "Happy Birthday."
Kerber will next play No 19 Ekaterina Makarova of Russia, who advanced after a tough 6-7 (4), 6-3, 6-4 win over 2007 Wimbledon finalist Marion Bartoli. In the fourth round last year, Makarova beat Serena Williams and was the first of only four women to beat the powerful American in 2012. Kerber was the last.
ASB Classic winner Agnieszka Radwanska, seeded fourth, won her 12th match in a row with a 6-3, 6-1 victory over Britain's Heather Watson.
Radwanska will now meet 2008 French Open champion Ana Ivanovic, who beat Jelena Jankovic 7-5, 6-3 in a matchup featuring two Serbians who were both formerly ranked No 1.
Stronger and fitter than in past years, Ivanovic again dominated compatriot Jankovic, extending her record to 8-3.
Sixth-seeded Li Na advanced 6-4, 6-1 over No 27 Sorana Cirstea of Romania and will next play No 18 Julia Goerges, who beat China's Zheng Jie 6-3, 1-6, 7-5.
Li won the 2011 French Open just a few months after reaching the final at the Australian Open.
- AAP, AP
Top results
• 1-Novak Djokovic (Serbia) bt 31-Radek Stepanek (Czech Republic) 6-4 6-3 7-5.
• 6-Li Na (China) bt 27-Sorana Cirstea (Romania) 6-4 6-1.
• 5-Angelique Kerber (Germany) bt WC-Madison Keys (US) 6-2 7-5.
• 4-Agnieszka Radwanska (Poland) bt Heather Watson (Britain) 6-3 6-1.
• 2-Maria Sharapova (Russia) bt 25-Venus Williams (US) 6-1 6-3.