Swiss teen ousts 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 smashing 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 blossoming career.

"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.

"I think I played really crappy today," Williams said. "I played like an amateur to be honest."

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 casts some 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 at times in 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. She won four straight to make it respectable in the third set but her comeback fell short.

"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."