World number 10 Caroline Wozniacki's preparations for the Australian Open were hampered after she was beaten by Kazakhstan's world number 103 Ksenia Pervak at the Brisbane International.
Wozniacki's defeat ended a year in which the Dane relinquished the number one ranking and failed to threaten at any of the major championships.
Pervak committed 61 unforced errors but still sealed a 2-6 6-3 7-6 (7-1) win.
Wozniacki said knee troubles had hampered her during the year.
"You know, I'm still 10 in the world," added Wozniacki, ahead of the Australian Open which begins on 14 January.
"Just the most important thing is that you're healthy. I had some struggles with my health. Obviously that makes a difference.
"But as long as I'm healthy and I can fight and be competitive, that is the most important thing."
In other first-round matches, number four-seeded Angelique Kerber of Germany opened with a 6-2 4-6 6-2 win over Anna Tatishvili of Georgia, French Open finalist Sara Errani beat Russia's Olga Puchkova 6-1 6-3 and American Sloane Stephens beat Slovakia's Dominika Cibulkova 6-2 6-3.
Alize Cornet of France advanced to a second-round match against third-seed Serena Williams with an opening 2-6, 6-3, 6-1 over Australian qualifier Bojana Bobusic, and Germany's Sabine Lisicki opened with a 6-2, 6-4 win over Lucie Safarova of Czech Republic to set up a second-round match against top-ranked Victoria Azarenka.
And Sweden's Sofia Arvidsson beat home favourite Samantha Stosur 7-6 (7-4) 7-5 to advance.
In the men's draw, fifth-seeded Kei Nishikori of Japan beat Australia's Marinko Matosevic 7-5, 6-2, sixth-seeded Florian Mayer of Germany beat Santiago Giraldo of Colombia 6-4, 6-4 and Jarkko Nieminen of Finland had a 6-3, 2-6, 6-1 win over Julien Benneteau of France.
Elsewhere, defending champion Zheng Jie and former world number two Svetlana Kuznetsova were knocked out of the Auckland Classic in the first round on Monday.