Venue: Melbourne Park Date: 19 January-1 February
Coverage: BBC Red Button, Radio 5 Live sports extra, BBC Sport website (Red Button coverage streamed on website throughout fortnight)
Azarenka was consoled by Williams after she was forced to retire
Serena Williams reached the last eight at the Australian Open when 19-year-old opponent Victoria Azarenka was forced to retire because of illness.
The Belarusian looked in great form early on but could barely stay upright when serving at 6-3 2-4, and left the court in tears.
Fourth seed Elena Dementieva beat Slovakia's Dominika Cibulkova 6-2 6-2 and faces Carla Suarez Navarro next.
Eighth seed Svetlana Kuznetsova went through when Jie Zheng retired.
The Chinese player, seeded 22nd, had earlier needed treatment on her left wrist and pulled out when trailing the Russian 4-1.
"I feel sorry for her because she was in great shape, she was playing good," said Kuznetsova. "This pain I hope is not very dangerous."
Dementieva cannot have expected her progress to be so simple against Cibulkova.
Bartoli (16) v Zvonareva (7)
Safina (3) v Dokic
Suarez-Navarro v Dementieva (4)
Kuznetsova (8) v S Williams (2)
The Slovakian led 2-1 against the Russian in the first set but lost 10 straight games.
Dementieva's win would have been even more straightforward had she not been broken while serving for the match at 5-0 in the second set.
That brief revival from Cibulkvova only delayed the inevitable, however, and a rasping backhand soon wrapped up Dementieva's win - her 14th straight victory.
The Russian later said she did not see herself as the title favourite.
"I don't think so, because Dinara (Safina) looks like she's in a good shape," she said.
"She's made the biggest improvement since last year, so she's probably the favourite to win," the 27-year-old said.
"But there is still Serena (Williams), and there are a lot of good players here. It's going to be a tough competition."
Dementieva faces Suarez Navarro in the last eight after the unseeded Spaniard beat Anabel Medina Garrigues 6-3 6-2.
The 20-year-old, who shocked Venus Williams in the second round, was too powerful for her compatriot, the 21st seed.
The world number 46, making her first appearance in the main draw at Melbourne Park, produced a string of winners from the back of the court.
Suarez Navarro sealed her victory with a thumping forehand down the line to reach her second Grand Slam quarter-final, following her appearance in the last eight of the French Open in 2008.
Williams had looked in serious trouble when Azarenka powered to the first set but the American was in charge of the second when her opponent retired.
"Honestly, I got worried," said Williams when asked about Azarenka's demise. "I was like 'Oh my goodness' because she started like walking really slow.
"The last thing she would want to do, to me, would be fall. It was just weird. I was actually really worried."
And Azarenka later explained: "I just woke up this morning feeling sick already, threw up all morning. Very disappointing for me.
"They think it was a virus or something. Whatever happens, happens. There was not much I could do about it."
Already through to the quarter-finals are Marion Bartoli, who sent top seed Jelena Jankovic crashing out on Sunday, Dinara Safina, Vera Zvonareva and Jelena Dokic.