MELBOURNE - Fired-up top seed Novak Djokovic powered into the Australian Open fourth round on Saturday, joining sizzling fellow world number one Serena Williams as Wimbledon champion Petra Kvitova was sent packing. The Serb, whose chances of a fifth Australian crown have vastly improved after Roger Federer's shock exit on Friday, ground down tenacious Spaniard Fernando Verdasco 7-6 (10/8), 6-3, 6-4.

He said he was highly motivated and over the virus that hampered his lead up to the season's opening Grand Slam. "He (Verdasco) was a former top-10 player. Somebody that loves playing on the big stage, a powerful game. I'm glad to go through in straight sets," he said. Williams, vying for a sixth Australian title and the first since 2010, was slow to find her groove against 26th-ranked Ukrainian Elina Svitolina, going down 6-4 in the first set.

But she shook off the cobwebs under the blazing sun on Rod Laver Arena to rattle through the next two sets 6-2, 6-0. It keeps alive her quest to add a 19th Grand Slam title, which would take her to clear second on the all-time Open Era Grand Slam winners list. She currently has 18 alongside Chris Evert and Martina Navratilova and is four behind Steffi Graf's 22. "When in doubt just start running as fast as you can, that's what Venus always told me, that helped," said Williams, whose world number one ranking is on the line if she fails to win the tournament.

She added that her elder sister's late-career renaissance was an inspiration. "She's winning, she's doing so well and I can do better. We always motivate each other. I'm so proud of her and we're so excited." Venus rallied from behind to defeat Camila Giorgi of Italy 4-6, 7-6 (7/3), 6-1 to make the second week of a Grand Slam for the first time since Wimbledon 2011. But there was heartbreak for Kvitova, a semi-finalist in 2012, as Lindsay Davenport-coached American teen Madison Keys pulled off a big 6-4, 7-5 upset.

"My hands are still shaking. Right now I can't even process this," said Keys, who set up an all-American fourth round showdown with Madison Brengle. Victoria Azarenka, the champion in 2012 and 2013 who is unseeded this year after an injury-marred 2014, also kept going, continuing her impressive comeback by downing Czech 25th seed Barbora Zahlavova Strycova. "There are things that obviously I want to improve. But it's progress. It's one step closer," said the Belarusian after her 6-4, 6-4 win which set up a clash with last year's beaten finalist Dominika Cibulkova.

Defending men's champion Stan Wawrinka, who beat Rafael Nadal in the final last year, could meet Djokovic in the semi-finals and stayed on track with a 6-4, 6-2, 6-4 victory over Finland's Jarkko Nieminen. The Swiss, who was always in control, was pleased with his progress.

 "It's been three really good matches, I think my game is there and I'm really happy to get through again," said Wawrinka, who will now meet Spain's Guillermo Garcia-Lopez. Eighth seed Canadian Milos Raonic also went through as did Japan's fifth seed Kei Nishikori.

It was a day to remember for the Williams clan with the evergreen Venus setting up a fourth round clash with dangerous Pole Agnieszka Radwanska. "That feels fantastic, especially when you have things happen in your life which are out of your control," said Venus, a seven-time Grand Slam winner who has had a long battle with the energy-sapping Sjogren's Syndrome.

Radwanska defeated American Varvara Lepchenko 6-0, 7-5 and has doled out a dreaded 6-0 "bagel" to each of her three opponents at Melbourne Park. She beat Venus in the Montreal final last year. "I think playing her is always a great challenge," said Radwanska, who has made the quarter-finals or better in Melbourne on her last four visits.  Serena will now play face Spain's Garbine Muguruza, who beat Switzerland's Timea Bacsinszky 6-3, 4-6, 6-0. The 24th seed ousted Serena at the French Open last year.