An erratic Madison Keys struck nearly 50 unforced errors on Thursday as the Australian Open champion crashed out of her final warm-up for this year's opening Grand Slam to teenager Victoria Mboko.
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The Canadian world number 17 withstood a second-set fightback from Keys to win 6-4, 4-6, 6-2 in the quarterfinal of the Adelaide International.
Keys won the Adelaide tournament 12 months on her way to lifting her maiden Slam title in Melbourne.
This year's Australian Open begins on Sunday and Keys has been drawn to face Ukraine's Oleksandra Oliynykova in the first round.
After dropping the first set, Keys rallied after twice being a break down in the second to the 19-year-old, before taking it to a deciding third set.
But Mboko broke the American twice for a 4-1 lead and won after 1hr 52min.
A year ago, Mboko was 337th in the world but she announced herself on the world stage last year by winning the WTA 1000 title in Montreal and the Hong Kong Open. .
"There were ups and downs in the match, I had to weather the storm," Mboko said.
"She won here last year so I know coming in this would be a big fight. I'm glad I was able to get it done."
"Madison is a tough player, she didn't get to where she is without a reason. It's nice to play a former champion to see what I can do."
Mboko will play a Friday semifinal against Australia's Kimberly Birrell, who came through a three-hour quarterfinal over Romanian Jacqueline Cristian 5-7, 6-1, 7-5.