The 19-year-old from Belarus was one of three players who had to withdraw from fourth-round matches on a sunny but mild day in Melbourne, with temperatures hovering around 75 degrees.

The second-seeded Williams was up a service break at 4-2 in the second set after No. 13 Azarenka had won the first set 6-3. Azarenka, serving at 30-30, wobbled back into the shade at the rear of the court, holding her face and choking back tears as she tried to regather herself.

She had needed a medical timeout earlier in the set and left the playing arena. She returned for 1 1/2 games but was unable to continue and was helped from the court soon after by two trainers.

Williams, seeking a 10th Grand Slam singles title, next plays 2004 U.S. Open champion Svetlana Kuznetsova, who advanced when Zheng Zie of China retired at 4-1 in the first set. No. 22 Zheng injured her left wrist when she tumbled after the third game. She had treatment immediately but retired two games later.

The all-French men's match between No. 6 Gilles Simon and his long-time friend Gael Monfils lasted only three sets. No. 6 Simon, who was leading 6-4, 2-6, 6-1 when No. 12 Monfils withdrew with a right wrist problem, reached the quarterfinals at a major for the first time.

The winners of the two completed women's matches meet in the quarterfinals.

Carla Suarez Navarro, the 20-year-old Spaniard who had an upset win over seven-time Grand Slam winner Venus Williams in the second round, beat No. 21 Anabel Medina Garrigues 6-3, 6-2.

She next plays Olympic gold medalist Elena Dementieva, the No. 4 seed from Russia who made the quarterfinals for the first time in 11 years at Melbourne Park with a 6-2, 6-2 win over Slovakia's Dominika Cibulkova.

Azarenka was taking the match to Williams, with both women going for winners constantly.

Williams was looking quicker on her feet than in her last two matches, but her first serve was another matter.

Clearly trying to concentrate, she regularly was bouncing the ball 20 times - she once hit 37 - before making her toss.

At 3-5 in the first set, Williams double-faulted for the second time to give Azarenka a set point, then loudly cursed her first serve, earning a warning from the umpire for an audible obscenity and sending a backhand long on the next point.

Things got better for Williams as she saved three break points in her opening serve of the second set in a game that went to deuce five times, then broke Azarenka to pull ahead at 2-1, shouting "Come on!"

Azarenka appeared to become ill after pulling within 3-2 and went off court. Williams took advantage of the time to get her ankles retaped.

Serena Williams, who has won the Australian title in 2003, 2005 and 2007, and Kuznetsova are the only major winners still in the women's draw.

Suarez Navarro is into the quarterfinals in her first trip to the Australian Open and second time overall, after reaching that round as a qualifier in the French Open.

The fourth-seeded Dementieva, who reached the finals at the French and U.S. Opens in 2004 but has not been to a Grand Slam championship match since, extended her winning streak to 14 matches. She won two titles in tuneup events.

Dementieva was ousted in the quarterfinals at the French last year and reached the semifinals at Wimbledon and the U.S. Open.

The top eight seeds in the men's draw made it to the fourth round here for the first time since 1970.

No. 1 Rafael Nadal, No. 4 Andy Murray and No. 5 Jo-Wilfried Tsonga were in action later Monday. If the top eight reach the quarters, it will be a first in a Grand Slam in the Open era.

Simon secured his place quickly Monday. No. 2 Roger Federer, No. 3 Novak Djokovic, No. 7 Andy Roddick and No. 8 Juan Martin del Potro all made it through on Sunday.

It was a tough win for Simon.

Monfils, a semifinalist at the French Open last year, had treatment during the third set, after going to the baseline to start the fourth, walked to the net and told Simon he could not continue.

The pair hugged at the net and kissed each other's cheek. Despite being close in age - Simon is 24, Monfils is 22 - it was their first match at tour level.