Serena Williams displayed all her fighting qualities to stay alive at the Australian Open on Monday along with defending champion Stan Wawrinka and Japan's Kei Nishikori, but two-time champion Victoria Azarenka was knocked out.
The world number one battled back from a set down in a tense match against Spain's Garbine Muguruza 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. I think I've been playing for a long time. When I have to go up a level, I have to."
Williams was seen coughing throughout the contest 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 elder sister Venus has a chance of joining them in the last eight, facing 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.
Wawrinka ensured his berth in the quarters by winning a thrilling fourth set tiebreaker to down Spain's tenacious Guillermo Garcia-Lopez 7-6 (7/2), 6-4, 4-6, 7-6 (10/8).
He will now play Japanese superstar Kei Nishikori, who was a class above dogged Spanish baseliner David Ferrer, easing past him 6-3, 6-3, 6-3.
"He's a tough, tough player. He improved a lot last year," Wawrinka said of Nishikori. "He's a great shotmaker, he can make winners. He doesn't give you a lot of time."
Former Australian champion Lindsay Davenport's protege Madison Keys made her first Grand Slam quarter-final, winning an all-American clash with Madison Brengle 6-2, 6-4.
The unseeded world number 35, just 19, has long been touted as the rising star of American tennis and will next play Venus Williams or Radwanska.
"I think I've matured a little bit and just got my game together a little bit more," Keys said, hailing the influence of Davenport.
Cibulkova, who lost in the 2014 title decider to Li Na, went into her match as an underdog against Azarenka, who had beaten her seven times in their nine previous meetings.
But she said her self belief blossomed as she stepped back onto Rod Laver Arena, the scene of her best Grand Slam performance.
"I just walked on the court and all the memories came to my mind and I was just thinking 'I'm a great player, I can do it, I just have to believe in myself'," she said after the fiercly-contested match.
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 bouts of depression last year.
"It's a good start. There was some good quality of tennis today, even though the result is not the one that I wanted and not what I was looking for.
"But overall, I can be pretty happy. But I'm such a perfectionist that I don't want to be satisfied."