MELBOURNE, AUSTRALIA - Serena Williams rode her luck to stay on track for her 10th Grand Slam title Monday as injury and sickness blighted the race to the Australian Open quarterfinals.

The American second seed was a fortunate winner in a tournament throwing up plenty of surprises when a tearful Victoria Azarenka retired ill while leading 6-3, 2-4 in their fourth-round match.

Azarenka, the 2005 Australian junior champion from Belarus, had broken Williams twice to win the first set and was down a break in the second when she became distressed on another scorching Melbourne day.

Williams was sympathetic.

"I just want to go inside and make sure she is OK. I feel so bad, she was playing so well," she told the crowd.

"She can't risk it so I hope she did the right thing. Obviously she was playing really well and she has so many Australian Opens ahead of her."

Williams now plays eighth seed Svetlana Kuznetsova for a place in the semi-finals, with the Russian also benefiting from a retirement.

Kuznetsova went through when China's Zheng Jie pulled out with a wrist injury while losing 4-1.

The result means 2004 US Open champion Kuznetsova equals her best performance at the Australian Open in 2005.

"I feel sorry for her because she was in great shape, she was playing good. This pain I hope is not very dangerous," she said of Zheng, who enjoyed her strongest run yet at the season-opening Grand Slam.

Looking ahead to Williams, she added: "It's definitely going to be hard."

"I mean, she's won many Grand Slams, she's very tough, she serves well."

"But definitely I'm confident in myself, and I just want to play my best and will go there with motivation, just fight with spirit hopefully."

Fellow Russian Elena Dementieva set up a quarterfinal against unseeded Spaniard Carla Suarez Navarro.

Dementieva is on a 14-match winning streak this year, with titles in Auckland and Sydney along the way, and has hardly been troubled.

She blasted past Slovak 18th seed Dominika Cibulkova 6-2, 6-2, with the 27-year-old launching a 10-game winning streak early in the first set, rating it her best performance at the year's season-opening Grand Slam.

"I'm very happy with my performance and glad to go through to the next round," said the lanky Russian.

Her next opponent, Suarez Navarro, reached her first Australian Open quarterfinals with a businesslike 6-3, 6-2 win over 21st seed Anabel Medina Garrigues.

Suarez Navarro, who earlier shocked Venus Williams, encountered little trouble, using her backhand to great effect as she broke Medina Garrigues twice in each set.

Among the men, sixth seed Gilles Simon also went through on a retirement as fellow Frenchman Gael Monfils pulled out with a wrist injury when losing 6-4, 2-6, 6-1.

It was not the way Simon wanted to beat his friend and must now regroup with either world number Rafael Nadal or dogged Chilean Fernando Gonzalez waiting in the quarterfinals.

"You never want to win like this. It's already strange when it's another player, but when it's a friend like Gael it's more difficult," he said.

"But I have a quarterfinal to play in two days, and I just have to focus on that."