If the rest of the men's field are beginning to get that sinking feeling, then blame Novak Djokovic.

While Roger Federer has already departed, beaten and bruised after a shock third-round exit at the hands of Italian journeyman Andreas Seppi and Rafa Nadal's stamina is in question as he returns from injury, the world No 1 rolls on.

Andy Murray may have something to say about the destiny of the title but after week one, it is Djokovic who looks the most likely, which is impressive considering he arrived in Melbourne still suffering from a virus picked up on his way through the Middle East.

Having won four titles here, including his first Grand Slam triumph in 2009, Melbourne is a home away from home for Djokovic and every year, he seems to hit top form quickly.

"It's a great confidence booster if you are on the court if you have great memories and you won the tournament four times," he said. "It's not [just] any tournament. It's a Grand Slam, one of the four most important events in our sport. I'm always trying to have that in the back of my mind, the great perform- ances I had over the years. I try to use that to my advantage."

Yesterday, Djokovic was in ruthless mood as he dismissed the challenge of Spain's Fernando Verdasco 7-6, 6-3, 6-4, a nice warm-up for a match with another left-hander, Gilles Muller of Luxembourg.

Muller, who is coached by former British player Jamie Delgado, took out John Isner yesterday but Djokovic said he would not be caught napping.

"He's been on the Tour for many years, best junior in the world," he said. "He's got a great serve, lefty. So I think the match tonight will help me in the next one, as well.

"He has a similar game, except he serves and volleys and he comes to the net. He has a nice slice serve. That's his favourite. He has struggled a little bit with injuries over the last couple of years, but I think the last six months he has been playing some of his best tennis. Now he's in the fourth round, winning against some top players. Winning against Isner in straight sets is never easy. He has done that. So he deserves respect. I need to be very alert and hopefully I can return well, which is going to be one of the keys of the match."

Djokovic is well aware of what is happening around him, which is just as well, for two of the biggest rivals in his half of the draw, Stan Wawrinka and Kei Nishikori, are also into the fourth round and looming large.

Defending champion Wawrinka reached the last 16 without dropping a set after a 6-4, 6-2, 6-4, win over Jarkko Nieminen.

The Swiss remains virtually unmentioned in terms of winning the title but having done it once, he is a massive threat to Djokovic and the rest.

But having only recently emerged from the shadow of Federer, Wawrinka still likes being under the radar and yesterday, stressed that there were many more likely winners.

"I've been feeling good since the beginning of the tournament but for sure I'm not the focus because there's Novak, Rafa [Nadal] coming back from injury," he said. "For me it doesn't matter. I'm feeling good. I'm feeling great. I'm happy the way I'm playing. To get to the second week again, it's great. So far everything is good for me. I still think Novak and Rafa are the big favourites for the tournament."

Fifth seed Nishikori, who beat Djokovic on his way to the US Open final last September, took his place in the last 16 with a 6-7, 6-1, 6-2, 6-3 win over American Steve Johnson.

Nishikori could face Wawrinka in the quarter-finals while Djokovic is seeded to face eighth seed Milos Raonic, the big-serving Canadian who seems to have improved perhaps more than anyone over the past year.

"I have a better understanding of how to deal with things, a better understanding of how to play second weeks," said Raonic, who beat Germany's Benjamin Becker 6-4, 6-3, 6-3. "The tournament really does start the second week of the slams. It's time to buckle down."