Ivanovic, the fifth seed, was dumped by qualifier Lucie Hradecka in the opening round of the year's first Grand Slam at Melbourne Park, which often proves a minefield for players short of match practice after the off-season.

Fellow seeds Angelique Kerber (9), Svetlana Kuznetsova (27) and Sabine Lisicki (28) were also among the day-one casualties, while China's Peng Shuai (21) and Carla Suarez Navarro (17) negotiated their way through.

Second seed Maria Sharapova of Russia and rising Canadian star Eugenie Bouchard, seeded seven, play later Monday.

Ivanovic's humiliating 1-6, 6-3, 6-2 loss to Hradecka of the Czech Republic was a disaster for the Serb, who won her only major at the French Open in 2008.

She was hoping to re-establish her Grand Slam credentials at the Australian Open, where she enjoys huge popularity among the home fans.

"It's really disappointing. It's probably the worst thing could happen," she said, admitting she felt "lost" on centre court as world number 142 Hradecka recovered from a woeful start to outclass a player 137 places above her.

Hradecka, who won a silver medal in doubles at the London Olympics and has three Grand Slam doubles titles, showed no signs of being intimidated, continually wrong-footing Ivanovic as she attacked the net.

"I still don't believe that it's true. Probably in a couple of hours I'll know that I am in the second round in the Australian Open and I beat Ana... I'm still smiling," she told reporters.

-- Early nerves --

World number three Halep also produced a stuttering first-up performance, with the Romanian admitting nerves almost got the better of her, but managed to regroup and down Italy's Karin Knapp 6-3, 6-2.

Despite the emphatic scoreline, Halep struggled to impose herself on world number 50 Knapp and was relieved to advance, saying nerves were always going to be an issue so early in a Grand Slam.

"I'm sure that next time I'll play better," said the 23-year-old, who is looking to build on a breakthrough 2014 when she reached the French Open final and the semi-finals at Wimbledon.

"I was thinking a lot - maybe too much - on Sunday about this match, (so) I'm really happy I could win this."      

Kerber of Germany lost to Irina-Camelia Begu of Romania 6-4, 0-6, 6-1.

It continued her unhappy run at the Australian Open, where she has exited in the first round three times and never advanced beyond the fourth round in eight attempts.

Meanwhile, Australian Jarmila Gajdosova finally broke a demoralising run at her home Grand Slam with a 6-3, 6-4 win over Romanian Alexandra Dulgheru, taking her through the opening round for the first time in a decade of trying.

Sharapova, the 2008 champion at Melbourne Park, will play Petra Martic of Croatia as she opens her campaign for a sixth Grand Slam title, with Bouchard facing Anna-Lena Friedsam of Germany.

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