Former world number one Caroline Wozniacki lost valuable Australian Open match practice when she was bundled out of the first round of the Brisbane International by Kazakhstan qualifier Ksenia Pervak on Monday.
In a titanic battle lasting 2hrs 48mins, left-handed Pervak eased past Wozniacki 2-6, 6-3, 7-6 (7/1) in the first shock of the season-opening WTA tournament.
Eighth-seeded Wozniacki had looked in control as she reeled off the first set in just 29 minutes.
But the Russian-born Pervak found her range in the second set and had the Dane constantly under pressure with her swinging left-handed groundstrokes.
She won the set to level the match then stunned Wozniacki in the deciding set with an all-out attack.
"I had a really tough match yesterday, we played for three hours and I was tired in the beginning," Pervak said.
"But then I got into the game (and) it was easy. I won a few games and I felt that I can do it and I started to fight more."
Pervak said she knew she could not give the more experienced Wozniacki any cheap points in the tiebreak.
"I just told myself that I needed to be focused on every point and just play my game as aggressively as I could," she said.
Wozniacki will play in Sydney next week in a last-ditch bid to find some form ahead of the January 14-27 Australian Open in Melbourne.
"Obviously you're a competitor so you want to win and you're not happy about it when you lose, no matter who you lose to or how you lose," she said.
"I fought until the end today. Maybe I didn't play my best tennis today, but it's tough to expect that from yourself in your first match back.
"Hopefully I can now play some practice matches with some of the other girls here and then go to Sydney and hopefully get a couple more there."
It was 21-year-old Pervak's first win over a top 10 player and the first time she has been past the first round in Brisbane in four attempts.
Fourth seed Angelique Kerber survived a tough three-setter against Georgia's Anna Tatishvili, winning 6-2, 4-6, 6-2, while German compatriot Sabine Lisicki had a more straightforward 6-2, 6-4 win over Czech Lucie Safarova.
French Open runner-up and fifth seed Sara Errani thumped Russia's Olga Puchkova 6-1, 6-3.
In the men's draw Japan's fifth seeded Kei Nishikori downed Australian number one Marinko Matosevic 7-5, 6-2, sixth seed Florian Mayer beat Santiago Giraldo of Colombia 6-4, 6-4 and Finland's Jarkko Nieminen outlasted Frenchman Julien Benneteau 6-3, 2-6, 6-1.
Second seeded Russian Maria Sharapova plays her opening match on Tuesday against Australia's Jarmila Gajdosova and said she was full of confidence heading into the new season.
"I'm feeling quite well. I had a really nice off season and just time to reflect on the year that passed," a relaxed Sharapova said.
"You go from tournament to tournament, and even when you achieve something like winning a grand slam you're not really given a chance to reflect on what you achieved.
"This was probably the busiest year for me in terms of match play and travelling, so having that chance to really reflect on the year and what you achieved and coming back from shoulder surgery and winning the French Open as my first grand slam (since surgery) was really nice."