Victoria Azarenka fends off Jamie Hampton in Australian Open

Third seed Serena Williams eased into the Australian Open fourth round yesterday, but defending champion Victoria Azarenka was made to work hard and play "ugly" to stay in the tournament.

Five-time champion Williams powered past Japan's Ayumi Morita 6-1, 6-3 after a minor second-set wobble, but it was much tougher for the top seed who survived a scare before beating injured American Jamie Hampton in three sets.

The Belarusian came through the match on Rod Laver Arena 6-4, 4-6, 6-2 to set up a clash with unseeded Russian Elena Vesnina, who upset 16th-seeded Italian Roberta Vinci.

But it was much harder than the scoreline suggested against a player who was wincing in pain from a back problem that needed treatment in the second set. "I don't think it was the best performance, honestly, today; but in another way I can say I can play better, so that's a positive note," said Azarenka, who beat Maria Sharapova in last year's final.

"You know, these kind of matches sometimes bring better things for you in the future because winning ugly always means that you overcame something, that you weren't feeling great and you weren't feeling all your shots. So I take it as a positive at the end of the day."

The plucky Hampton had never played a grand slam third round before, and gave it her all in a match packed with quality rallies and tight games.

On a cool Melbourne day, Azarenka was given a thorough test and was soon pumping her fist and shouting "C'mon", as her emotions showed.

Neither player was serving well and Hampton left the court for treatment on a lower back problem.

But despite being in obvious pain she returned to take the second set, with Azarenka slamming her racquet in frustration.

The brave American, who appeared close to tears at times from the discomfort, had the crowd onside, but Azarenka fought back to win the match and stay on course to meet Caroline Wozniacki in the quarter-finals.

It was much easier for Williams, who followed Azarenka onto centre court.

With sister Venus out of the tournament after being crushed in straight sets by Sharapova in the third round on Friday, she was in no mood to be detained longer than necessary.

There was no stopping her in the first set, with Williams firing down the fastest women's serve of the tournament so far at 207km/h.

But the 15-time major winner, who has an eye on the first calendar-year grand slam since 1988, wavered on her serve in the second set and Morita raced to a 3-0 lead.

It was a shortlived advantage with Morita imploding and Williams reeling off six games in a row to easily clinch the match.

"I was really focused out there," said Williams, who now faces 14th-seeded Russian Maria Kirilenko, who beat Yanina Wickmayer of Belgium 7-6 (7-4), 6-3.

"Ayumi played well and she gave me a good test."

In other matches, former US Open and French Open winner Svetlana Kuznetsova of Russia beat Spaniard Carla Suarez Navarro in three sets and will next play Wozniacki, who beat Ukrainian Lesia Tsurenko in two sets.

Young American Sloane Stephens, 19, won the battle of the teen brigade when she sent Britain's Laura Robson, 18, packing 7-5, 6-3, while Japan's Kimiko Date-Krumm, 42, also lost. The Japanese veteran was beaten 6-2, 7-6 (7-3) by Serb Bojana Jovanovski, 21, who was not even born when Date-Krumm made her grand slam debut in 1989.