MELBOURNE  - Novak Djokovic and Andy Murray hit top gear on Thursday as warhorse Lleyton Hewitt capitalised on Andy Roddick's retirement to reach the third round.

Serena Williams was also at her destructive best with Petra Kvitova the only top seed given trouble on day four. And Hewitt was handed easy passage when Roddick strained a hamstring, ending the clash of the old stagers.

"It's a nightmare for both of us," said Hewitt, who was leading the night match 3-6, 6-3, 6-4 when Roddick called it quits. "He stretched something high in his leg and it was hard for me to concentrate at the other end.

"You know Andy is hurting and you try to block it out of your mind and it's hard to do. Without sounding too mean, you try to run him around a lot more."

The one-time world number one, still battling on despite five major operations in the past four years, now faces another big server in Canada's Milos Raonic.

But the despondent Roddick, who tried to play on after injuring his right hamstring tendon in the second set, said he would probably be out for three weeks.

"You can try to ham and egg it against a lot of guys. But he's really intelligent. He knew what was going on," said the American.

"Then you're out there and you're wondering, listen, even if this goes your way, you're not going to play in two days. So it's a miserable, terrible thing being out there compromised like that. It really sucks."

Djokovic has been in commanding form at the year's first major as he bids to repeat last season's astonishing haul of three grand slams among 10 titles, with a 41-match winning streak along the way.

And the Serb expended little energy in his 6-3, 6-2, 6-1 hit-out against Colombia's Santiago Giraldo as he speeds towards an expected semi-final with Murray, his victim in last year's title match.

"I try to not underestimate any opponents in early rounds," Djokovic said.

"Santiago came out early hitting the ball quite flat. But I knew that sooner or later he's going to drop the rhythm and I just have to hang in there. I've done a good job," he added.

Two-time finalist Murray, whose new coach Ivan Lendl sat swathed in a towel against the evening chill, raced through the first set in just 23 minutes as he beat France's Edouard Roger-Vasselin 6-1, 6-4, 6-4.

Williams brought up her 500th career win, against Barbora Zahlavova Strycova, to keep up her record of reaching the third round in every grand slam she has entered apart from her debut, in Melbourne in 1998.

"It's fine, I just have really wobbly ankles," she said, after falling heavily in the last game. "I wasn't meant to be a ballerina."

Second seed Kvitova had a trickier assignment against Spain's Carla Suarez Navarro, but she kept her frustrations in check to survive a mid-match lapse 6-2, 2-6, 6-4.

Meanwhile former champion Maria Sharapova made it two wins for the loss of just two games, and 2008 French Open winner Ana Ivanovic also strolled through to the weekend round.

Resurgent Sharapova, the 2008 winner, made light of her total lack of competitive tennis this year as she thrashed US qualifier Jamie Hampton 6-0, 6-1, after dispatching Giselo Dulko by an identical score in the first round.

Former finalist Jo-Wilfried Tsonga went through in straight sets against Ricardo Mello but fifth-seeded baseliner David Ferrer, who reached the semis last year, needed five to beat American Ryan Sweeting.

Gael Monfils ousted Brazil's Thomaz Bellucci, Julien Benneteau dispatched fellow Frenchman Gilles Simon, and Canada's Milos Raonic wielded his huge serve as he downed Germany's Philipp Petzschner 6-4, 5-7, 6-2, 7-5.

Among the other women's results, China's Zheng Jie upset Italian seed Roberta Vinci in straight sets and Sara Errani accounted for Nadia Petrova.

And players shrugged off Marcos Baghdatis's extraordinary tantrum, in which he smashed four racquets during his loss to Stanislas Wawrinka late on Wednesday and was fined US$800.

"You know, sometimes you get angry and it's difficult to control yourself. But, I mean, one, it can happen. Four, it's a lot," smiled Tsonga.