Melbourne, Australia - The Williams sisters are through to the fourth round at the Australian Open, but both top seed Serena and No. 18 seed Venus had to rally from a set down Saturday.

Serena, a five-time Aussie champion, came back to down 26th-seeded Ukrainian Elina Svitolina, 4-6, 6-2, 6-0. Venus, a seven-time Grand Slam champion, topped talented Italian Camila Giorgi, 4-6, 7-6 (7-3), 6-1.

Meanwhile, two-time Wimbledon champion Petra Kvitova's struggles continued at the year's first major championship. American Madison Keys pulled off a 6-4, 7-5 upset, sending the fourth-seeded Kvitova to an early exit for the fifth time in her seven visits to Melbourne.

It was a rough start for Serena, who fell behind 5-2 in the first set, but then won the next two games. Svitolina came back to hold at love the next game to take the set.

The final two sets were completely different, as the 18-time Grand Slam champion and reigning U.S. Open queen won the first four games of the second and throttled her opponent in the third.

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

There was a brief opening for Svitolina in the third game of the final set, but she blew a pair of break point chances.

Revenge could be a factor in Serena's next match. She was the defending champion headed into the 2014 French Open, but lost in the second round to Spain's Garbine Muguruza.

Muguruza, the 24th seed here, set up her match with Serena with a win over Switzerland's Timea Bacsinszky, 6-3, 4-6, 6-0.

"It was a good loss," Serena said of her loss to Muguruza. "As angry as I was, it was the best loss I had the whole year last year. Had a lot of them. But that one in particular made me realize what I needed to work on. It opened my eyes towards a lot of things."

Venus was two points away from losing the match to Giorgi, but then rallied. Giorgi double-faulted 16 times.

This will be the first time Venus has reached the round of 16 at a major since Wimbledon 2011.

"It's definitely been a lot of work and a lot of learning and a lot of perseverance," Venus said. "It will continue to be that for me. Just have to come to terms with it."

Next up for Venus is sixth seed Agnieszka Radwanska of Poland. The Wimbledon runner-up surged past American Varvara Lepchenko, the 30th seed, 6-0, 7-5.

Four Americans are alive in the top half of the draw, as Keys and Madison Brengle will join the Williams sisters in the fourth round.

Keys, coached by three-time Grand Slam champion Lindsay Davenport, came up with the biggest victory of her young career with the stunner against Kvitova. She broke serve twice in the first set and three times in the second, the last for a 6-5 lead before holding to finish the match in an hour and 24 minutes.

"I think my hands are still shaking," said the 19-year-old Keys after the match. "I'm really happy that I served that (last) game out so well."

Keys will next play Brengle, a 6-3, 6-2 winner over fellow American Coco Vandeweghe.

Kvitova has been past the third round just twice at this event, reaching the quarterfinals in 2011 and the semifinals in 2012. She exited in the first round last year after a second-round setback the year before.

"In the end of the day I did better than the last year and the year before, so I need to be happy, but I'm not," said Kvitova. "I expect little bit more from this tournament unfortunately."

Two-time Australian Open champion and former world No. 1 Victoria Azarenka defeated 25th-seeded Barbora Zahlavova Strycova, 6-4, 6-4. Unseeded this year, Azarenka will next play 11th-seeded Dominika Cibulkova, a 7-5, 6-2 winner over Frenchwoman Alize Cornet.

Fourth-round play begins Sunday, featuring second-seeded Maria Sharapova and third-seeded Simona Halep. Sharapova will take on China's Peng Shuai and Halep will open the night session at Laver Arena against Belgium's Yanina Wickmayer.

Also Sunday, seventh-seeded Canadian Eugenie Bouchard will face Romania's Irina-Camilia Begu to start the day and 10th-seeded Russian Ekaterina Makarova will meet Germany's Julia Goerges.