Top seed Caroline Wozniacki was bundled out of the WTA Auckland Classic in the second round Thursday, beaten in straight sets by 18-year-old Canadian Bianca Andreescu.
It was a blow for Wozniacki who was looking for as much court time as she could get in Auckland on her way to defending her Australian Open crown.
She was beaten 6-4, 6-4 by the Canadian teenager who is ranked 152nd in the world and had to play her way through the qualifying tournament to make the main draw.
"Honestly, I can't believe it," said Andreescu who was near speechless at having beaten Wozniacki to set up a quarter-final against Venus Williams who progressed to the final eight with a 6-4, 6-3 win over fellow American Lauren Davis.
"First I have to swallow this one and then I'll think about tomorrow. But I'm honestly really excited just being here.
"I've dreamt of playing on this stage against hard players and now I'm here I played against one of the best and I won so really, I don't know what to say.
"I managed to pull through. I've no idea how. I just fought to the end."
Andreescu played herself to a standstill, keeping rallies alive with a mix of well-angled strokes from the baseline and delicate drop shots until Wozniacki made mistakes.
Wozniacki did produce glimpses of her best and determinedly levelled at 4-4 in the first set when she took Andreescu's serve in a 12-minute, seven-deuce game.
But the Canadian teen immediately broke again and held serve for the set before breaking Wozniacki twice in the second set.
The result would have surprised Williams who had earlier spoken of how she expected to meet Wozniacki in the quarter-finals.
"That'll be a great match up. Finals worthy but it's the quarter-finals and we'll battle it out," Williams had said as she looked ahead after beating Davis.
Williams, seeded six in Auckland, broke Davis for the first time in the sixth game of the first set and took control from there.
On a day dominated by teenagers, 17-year-old American Amanda Anisimova stunned fifth seed Barbora Strycova 6-3, 6-3.
Anisimova, ranked 96th in the world and the youngest player in the top 100, never dropped a game against the Czech veteran and cashed in on all three break points that went her way.
Anisimova will play big-serving Viktoria Kuzmova from Slovakia in the quarter-finals after Kuzmova fired down 20 aces when dispatching American Sofia Kenin 7-5, 5-7, 6-3.
In the other quarter-finals, second seed Julia Goerges plays Eugenie Bouchard and third seed Hsieh Su-wei is up against Sara Sorribes Tormo.