MELBOURNE: Fired-up top seed Novak Djokovic powered into the Australian Open fourth round on Saturday, joining sizzling fellow world number one Serena Williams and injury-free two-time winner Victoria Azarenka.

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.

“Physically, I feel fit now and I'm always motivated to play well in Australia,” said Djokovic.

Williams, vying for a sixth Australian title and the first since 2010, was slow to get in the 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 German great 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.