World number one Andy Murray suffered a huge upset at the Australian Open on Sunday, opening the door wider to Roger Federer who edged closer to an 18th major title.

Murray was the hot favourite after Novak Djokovic's shock exit, but the Briton lost 7-5, 5-7, 6-2, 6-4 to serve-volleyer Mischa Zverev to extend his wait for a first Melbourne title.

Federer's chances of a first major championship since 2012 are improving all the time, with Zverev his next challenge after needing five sets to down Kei Nishikori 6-7 (4/7), 6-4, 6-1, 4-6, 6-3 in round four.

"This is a huge moment for me in my career," said the 35-year-old Swiss who already holds a record 17 Grand Slam victories.

"I was telling myself, 'Stay calm'. I feel fit, I've trained as hard as I possibly can in the off-season. That's what you tell yourself and you hope it pays off," he added.

Murray was never expected to be troubled by Germany's Zverev, 29, who has never won an ATP title, but he lost in his earliest Melbourne departure in eight years.

It isn't since the 2004 French Open that the top two seeds have gone out before the quarter-finals, a development that spells opportunity for the remaining players.

With the defeat, Murray extends his curse at the Australian Open, where he has been runner-up five times -- losing four finals to Djokovic -- without lifting the trophy.

He also made the earliest exit for an Australian Open top seed in 14 years, since Lleyton Hewitt fell at the same stage in 2003.

"I've had tough losses in my career in the past. I've come back from them. This is a tough one," Murray said.

"I'm sure I'll come back okay from it. But right now I'm obviously very down because I wanted to go further in this event, and it wasn't to be."

- 'A little coma' -

Zverev, a former junior rival of Murray's, stretched to a series of elastic volleys to halt the nonplussed Scot, coming to the net 118 times.

"I was like in a little coma, just serving and volleying my way through it. There were a few points where I didn't know how I pulled it off but somehow I made it," he said.

Zverev, a late bloomer after a career riddled with injuries, reaches his first Grand Slam quarter-final where he will play his idol Federer on Monday.

Meanwhile Stan Wawrinka, who won the first of his three Grand Slam titles in Melbourne in 2014, came through 7-6 (7/2), 7-6 (7/4), 7-6 (7/4) against Italy's Andreas Seppi.

The formidable Swiss, now into his fourth Australian Open quarter-final, will play France's Jo-Wilfried Tsonga, who came from a set down to beat British surprise package Dan Evans.

"It's a big surprise for sure," Wawrinka said of Murray's defeat. "It shows that it's not so easy as we can think, or when we see the draw, to just win, keep winning all the time."

Earlier Venus Williams, the oldest player in the women's draw at 36, also became one of the first into the quarter-finals when she beat German qualifier Mona Barthel 6-3, 7-5.

The seven-time Grand Slam winner allowed herself to dream of a possible final with her sister Serena, who is steaming through the other side of the draw in search of her 23rd major title.

"That could hopefully happen. We both still have to work very hard to get there," Venus Williams said.

Her quarter-final opponent is Anastasia Pavlyuchenkova, who beat fellow Russian Svetlana Kuznetsova 6-3, 6-3 to reach her first Australian Open quarter-final in nine attempts.

French Open champion Garbine Muguruza beat Sorana Cirstea 6-2, 6-3 to set up a last-eight encounter against top seed Angelique Kerber or Coco Vandeweghe.

source: AFP