Serena Williams admitted that a bad back sustained during practice hampered her movement during her fourth-round loss to Ana Ivanovic on Sunday, but refused to blame the injury for her stunning exit that blew the Australian Open wide open.

An overwhelming favorite to clinch her 18th Grand Slam title and sixth at Melbourne Park, the world No 1 was overhauled 4-6, 6-3, 6-3 in front of an astonished Rod Laver Arena crowd as the resurgent Ivanovic soared to her finest victory in years.

Williams crashed out in a hail of 31 unforced errors and managed only three winners in the third set as the anticipated fight back the American has counted on throughout her career failed to materialize.

Her coach, Patrick Mouratoglou, who talked up a serious tilt at a calendar Grand Slam before the tournament, said Williams had "blocked" her back during practice before her third-round match against Daniela Hantuchova.

"I guess the secret is out, but I obviously wasn't hitting the way I normally would hit and wasn't moving the way I normally would move and making a lot of errors that I normally would not make and I haven't made in a couple of years," she said after her 25-match winning streak was snapped.

"But it's OK. I feel like I know for a fact I can play so much better than what I did today, so with that, knowing that, I'm not disappointed or anything. I just know that I can play 10 times better than what I did today.

"I have just been on some of the strongest meds I can take, that are legal, which my body didn't handle it well because I haven't been taking anything. So I was like really kind of out of it for a few days."

The back injury makes three straight Melbourne Park campaigns where Williams, the greatest player of her generation, has suffered untimely fitness problems to exit relatively early.

She was knocked out last year in the quarterfinals by Sloane Stephens after rolling an ankle early in the tournament and was stunned in the fourth round by Ekaterina Makarova in 2012.