Swiss teen Belinda Bencic beats Serena Williams

World number one Serena Williams was sent crashing out of the WTA Rogers Cup after being upset by Swiss teen Belinda Bencic in a three-set marathon semi-final on Saturday.

The 33-year-old Williams, a 21 Grand Slam winner, suffered just her second loss of 2015 as the oldest and youngest players in the draw clashed in a tense two-hour, 28-minute showdown.

Bencic, 18, stormed from behind to win 3-6, 7-5, 6-4, snapping Williams' 14-match win streak in Toronto to book her spot in Sunday's final against second seed Simona Halep.

Bencic closed out the contest on her first match point by hitting a forehand winner to the open court. Bencic bent over and covered her face with both hands as she soaked in the biggest victory of her young career.

"I can't describe the feeling right now," said Bencic.

"I was very overwhelmed from the situation on the court, but I'm just so happy the forehand landed in and she couldn't reach it anymore.

"It was an incredible feeling. I have no words."

Bencic is the youngest player to beat Williams since the American lost to a 17-year-old Maria Sharapova in the final at Wimbledon in 2004. Bencic, who also has Slovak citizenship through her parents, was just two years old in 1999 when Williams won her first major title.

"She never gives up and she fights hard," Williams said of Bencic.

Bencic has won 20 of her last 24 matches, beating two top 10 ranked players (No. 5 Caroline Wozniacki and No. 6 Ana Ivanovic) to reach the semis.

The loss throws a shadow of doubt into Williams' pursuit of her fifth Grand Slam title in a row as she attempts to complete a calendar year Grand Slam by defending her title at the US Open, which begins August 31.

A US Open crown would give her 22 Grand Slams, matching Steffi Graf for the most in the Open era.

She pulled out of the Stanford tournament last week with an elbow injury but said after her quarter-final victory in Toronto that it was no longer a problem.

Williams had won in Toronto in her last two appearances in 2011 and 2013.

But she looked sluggish and troubled throughout much of the semi-final, failing to run down balls in the open court and often turning in frustration towards the stands to speak with her coach Patrick Mouratoglou.

Despite the defeat Williams said she felt she was in charge of the points. 

12 double faults

"I felt pretty much in control until I lost the match," said Williams, who had 12 double faults. "I always felt I still had an opportunity to stay in there. And obviously I was down and stuff. But I just wasn't playing well today."

What she wasn't in control of at times was her emotions. Williams had a breakdown in the second set, smashing her graphite racquet to pieces after she double faulted on game point to go down 5-3.

And Bencic kept Williams off balance with precision ground strokes and a neat serve-and-volley game when she needed it.

Halep advanced with a 6-4, 6-4 win over Italy's Sara Errani. The Romanian second seed Halep will play for her 12th career title and fourth of the year.

Halep's earlier titles this season came in January at Shenzhen, February at Dubai and in March at Indian Wells.

"Obviously Halep is No.3 in the world and she's an amazing player," Bencic said.

"But I'm in the final and everything that comes is going to be a bonus."

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