Belinda Bencic stunned top-seeded Serena Williams 3-6, 7-5, 6-4 on Saturday night to advance to the women's final at the Rogers Cup.
The Swiss teenager took advantage of several uncharacteristic errors from the veteran American, who struggled with her serve most of the evening.
It's just the second loss of the season for Williams, who falls to 43-2 on the year.
Bencic, the unseeded world No. 20, has knocked off three seeded players en route to her first career appearance in a Premier-5 Level event.
The 18-year-old opened with a victory over Canada's Eugenie Bouchard and followed with wins over fourth-seeded Caroline Wozniacki, Sabine Lisicki and fifth-seeded Ana Ivanovic.
Bencic will play second-seeded Simona Halep of Romania in Sunday's final. Halep beat Italy's Sara Errani 6-4, 6-4 in the other semifinal.
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