Williams was seen coughing throughout the match and confessed that "I've been a little sick in the last couple of days".
Serena Williams displayed all her fighting qualities to grind down the dangerous Garbine Muguruza and stay alive at the Australian Open Monday, but two-time champion Victoria Azarenka was bundled out.
The world number one battled back from a set down in a tense centre court match against the 24th seeded Spaniard to win 2-6, 6-3, 6-2 and make the quarter-finals, staying on track for a 19th Grand Slam title and her sixth at Melbourne.
"I didn’t start out so well and she did everything she needed to do in the first set," said Williams, who can lose her top ranking if she exits and Maria Sharapova wins the tournament.
"I just decided to do my best, stay focused and relax."
Williams was seen coughing throughout the match and confessed that "I’ve been a little sick in the last couple of days".
She will next face last year’s finalist Dominika Cibulkova, the 11th seeded Slovak, who ended the comeback from injury of Azarenka in three tight sets.
Her sister Venus has a chance of joining them in the last eight, facing gritty Polish sixth seed Agnieszka Radwanska in a night match with the evergreen 34-year-old in the second week of a Slam for the first time since Wimbledon 2011.
World number one Novak Djokovic takes on unseeded Gilles Muller of Luxembourg later Monday with the winner playing Canadian eighth seed Milos Raonic or Spanish 12th seed Feliciano Lopez in the last eight.
Defending men’s champion Stan Wawrinka meets Spaniard Guillermo Garcia-Lopez with the victor battling it out with Japanese superstar Kei Nishikori or Spanish terrier David Ferrer for a spot in the semi-finals.
Despite her disappointment Azarenka said she was content with what she had achieved so far this year on the comeback trail, including a defeat of eighth seed Caroline Wozniacki in round two.
"I take it as progress. I think there are a lot of the positive things to take from here," said Azarenka, who won the Melbourne title in 2012 and 2013 but struggled with a string of foot injuries and depression last year.