Williams sisters into Last 16

Melbourne - Top seed Serena Williams powered into the Australian Open fourth round on Saturday, but only after sister Venus inspired her to shake of a sluggish start against Ukrainian 26th seed Elina Svitolina.

The 18-time Grand Slam champion conceded her first set of the tournament to Svitolina, continuing a run of slow starts, before Venus' hard-fought run into the final 16 at Melbourne Park convinced to her to knuckle down.

The American cruised through 4-6, 6-2, 6-0 when she finally focused, but said she did not know why she continued to zone out as she chases a sixth Australian title with her world number one ranking on the line.

"I'm not sure. I need to figure that out," said Williams, who faces dangerous Spanish 24th seed Garbine Muguruza in the fourth round.

"My next match I'm just going to have a longer warm-up, a more intense warm-up. But as long as I was able to come through today, I can always have an opportunity for tomorrow."

The 33-year-old said there was little she could do but hang on early in the match against an inspired Svitolina.

"She kept hitting winners in the first set, there wasn't much I could do," Williams said. "When in doubt just start running as fast as you can, that's what Venus always told me."

In the end, it was Venus who provided the inspiration for her to hit the gas with a back-from-the-dead performance of her own to make the last 16.

She said that while playing she kept an eye on a scoreboard showing Venus' progress in her three-set win over Italy's Camila Giorgi, drawing motivation from her sister's advance.

"(I thought) 'C'mon Serena, she's winning, she's doing so well and I can do better', you can do it too," she said.

"She's been through so much with her illness, with everything that she's had to do. Gosh, if she can do it, I'm perfectly healthy, I'm fine. I should be able to do it, too."

Williams was full of praise for Svitolina, describing the 20-year-old who was playing at Melbourne Park for only the second time as "one to watch".

The big-serving youngster caught Williams napping and was soon up two breaks, with the world number one showing the same inaccuracy and unforced errors that dogged her earlier in the tournament.

She broke back and tried to get herself out of trouble in the next game with an ace but Svitolina refused to be intimidated.

The pair exchanged breaks again before the Ukrainian served out the set after 36 minutes.

The American was vastly improved in the second set, blasting down five aces including a 196 kilometres (122mph) thunderbolt, gradually seizing momentum off Svitolina to take it 6-2 after 35 minutes.

The Ukrainian's serve began to fall apart with a string of double faults and her resistance finally crumbled in the second game of the third set after losing a 23-shot rally to put Williams up a break.

Williams then took full control to hold Svitolina to love in the third set.

She was wary of Muguruza, who defeated her in last year's French Open, saying the Spaniard had played some "miraculous" tennis at Roland Garros..

"She has the potential to want to do that again against me. So I'm going to be ready for that," she said.