Toronto: Swiss teen Belinda Bencic upset world number one Serena Williams by storming back for a 3-6, 7-5, 6-4 victory in the semi-finals of the WTA Rogers Cup on Saturday.

Williams, a 21-time Grand Slam winner, suffered just her second loss of 2015 to the 18-year-old Bencic as the oldest and youngest players in the singles draw met in a tense two hour, 28 minute thriller.

Bencic advances to Sunday’s final, where she will face Simona Halep, who booked her spot with a 6-4, 6-4 win over Italy’s Sara Errani at the hardcourt warm-up event for the US Open that begins on 31 August.

Bencic is the youngest player to beat Williams, 33, since the American lost to a 17-year-old Maria Sharapova in the final at Wimbledon in 2004.

Bencic, who has Slovak citizenship through her parents, closed out the contest on her first match point when Williams hit a shot long. Bencic bent over and covered her face with both hands as she soaked in the biggest win of her young career. 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 US Open crown in New York.

A US Open title would give her 22 Grand Slam titles, 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 win here that it was no longer a problem.

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

She looked sluggish and troubled throughout much of Sunday’s 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.

Williams had a breakdown early in the second set, smashing her racquet to pieces after she double faulted on game point to go down 5-3.

Bencic kept Williams off balance with precision ground strokes from the baseline and a superb serve and volley game when she needed it.

Williams was also at times her own worst enemy, finishing with 12 double faults.

Romanian second seed Halep will play for her 12th career title and fourth of the year.

It was the third win in four tries for Halep over Errani, having lost their first matchup at Miami in 2013 but won last year in a Doha quarter-final and earlier this year at Stuttgart.

‘I didn’t play my best tennis but I played well enough to win the match,’ Halep said. ‘I knew she would play great. I tried everything. I was fighting until the end.’

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