World number one Victoria Azarenka and five-time champion Serena Williams took contrasting paths to the fourth round of the Australian Open today.

Azarenka came through an epic struggle with injury-stricken Jamie Hampton while Williams had a far easier time of it, outclassing Ayumi Morita.

First on to Rod Laver Arena was Azarenka and she endured plenty of nervy moments, turning in a scatter-gun display, before coming through 6-4 4-6 6-2.

Indeed, the match was in the balance before Hampton suffered a recurrence of a lower back injury - later confirmed to be two herniated discs - which hampered her movement late in the second set.

Azarenka admitted she had been impressed by Hampton on their first meeting. She said: "She played incredibly. She went for every single shot.

"All credit to her, she has a great future in front of her, she has great potential."

The Belarusian did, however, feel there were positives in her victory.

She added: "These kind of matches are sometimes better because winning ugly always means you have overcome something when you weren't feeling great, weren't feeling all your shots.

"It's always good to know you can battle through when you're not playing well."

Azarenka almost blew a 5-1 first-set lead, twice squandering the chance to serve it out before finally edging ahead as Hampton wobbled.

To her credit, Hampton responded well as Azarenka continued to look fragile.

She established a 3-1 lead in the second set and maintained the advantage before bringing up two set points on the Azarenka serve at 5-3.

But the nerves again got to her as she made two basic mistakes, allowing Azarenka to escape immediate danger.

More seriously for Hampton, though, was the fact she suffered the back problem in hitting a forehand and had to call for the trainer.

She returned bravely from a medical time-out to serve out the set but the problem would not go away - and, indeed, appeared to get worse.

Grimacing after every shot, Hampton remarkably broke to go 2-1 up but, by now also suffering from cramps in her legs, she could not win another game as Azarenka took full advantage to go through. Next up for the top seed is a meeting with Elena Vesnina, who beat Roberta Vinci.

Williams' only concern was a minor second-set lapse - a problem she quickly rectified.

The third seed found herself 3-0 down but responded in the style of a champion to rattle off six games in a row to advance 6-1 6-3.

In further positive news for Williams, she showed no lingering signs of the ankle injury suffered in her first-round whitewash of Edina Gallovits-Hall and which appeared to hamper her movement in Thursday's victory over Garbine Muguruza.

"I feel good," she said. "I was really focused today, she played well and it was a good test."

Williams may see it that way but, in truth, the outcome was never in doubt.

The American ran through the first set with her weight of shot too much for Japanese number one Morita.

Williams broke in games four and six and was rarely troubled on her own serve.

Morita stunned everyone inside Rod Laver Arena by taking the first three games of the second but, seemingly affronted, Williams moved up a couple of gears to extend her winning streak to 19 matches and set up a fourth-round clash with Russian Maria Kirilenko.

Kirilenko beat Yanina Wickmayer 7-6 (7/4) 6-3 while Svetlana Kuznetsova is also through to a meeting with Caroline Wozniacki following a 6-2 4-6 6-3 defeat of Carla Suarez Navarro.

Former world number one Wozniacki beat Ukrainian qualifier Lesia Tsurenko.

Kimiko Date-Krumm's run in Melbourne is over after the 42-year-old went down 6-2 7-6 (7/3) to Bojana Jovanovski, who will now take on Sloane Stephens in the last 16. Stephens saw off fellow teenager Laura Robson in straight sets.