MELBOURNE. Serena Williams admitted that a bad back sustained during practice hampered her movement during her shock 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.Raging favorite to clinch her 18th grand slam title and sixth at Melbourne Park, the world number one 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 fightback 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 told reporters after her 25-match winning streak was snapped.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.Williams had never lost a set to Ivanovic, let alone a match, before Sunday and since last year’s Australian Open was riding a 77-3 winning record.Pundits had talked of the American’s 18th grand slam title being a formality -- barring injury or illness -- and debated how soon the 31-year-old would reach Steffi Graf’s total of 22 grand slam titles.With Williams gone, 20-year-old Stephens is the last American woman in the singles draw. It also leaves a cavernous hole in the top half of the draw, where 14th seeded Ivanovic and China’s fourth seed Li Na are the hightest-ranked players.