TORONTO - One by one, Belinda Bencic has taken on the big guns of the WTA Tour at the Rogers Cup.

Canadian Eugenie Bouchard was first up and first down. Fourth-seeded Caroline Wozniacki was next to fall and Sabine Lisicki and No. 5 Ana Ivanovic soon followed.

With those impressive wins under her belt, the Swiss teenager had nothing to lose Saturday against Serena Williams.

Bencic came through yet again with a performance she'll never forget, stunning the top-ranked American 3-6, 7-5, 6-4 for the biggest win of her career.

"I can't describe the feeling right now," Bencic said. "I was very overwhelmed from the situation on the court, but I'm just so happy the (winning) forehand landed in and she couldn't reach it anymore.

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

Williams was hamstrung by service problems and inconsistent play in the evening match on Centre Court. She has been dominant this season — winning the first three Grand Slams this year — but a rare off-night left her with a 43-2 mark on the season.

"I think I played really crappy today," Williams told reporters. "I'll try to be politically correct. And I don't think you would disagree."

Bencic will play second-seeded Simona Halep in Sunday's final at the US$2.51-million tournament. Halep beat 15th-seeded Sara Errani of Italy 6-4, 6-4 in the other semifinal.

With nerves and tension building, Bencic nearly let the third set slip away. She stormed out to a 5-1 lead but Williams picked up two breaks — sealing the second one on a double-fault — to get back on serve.

"I was very nervous in that moment," Bencic said. "But I told myself today is my chance and I really have to stay focused and be mentally tough, even if I didn't do it on the first try."

Bencic didn't quit despite her opponent's renewed focus. In the final game, she delivered a driving backhand that Williams couldn't return to get to match point and delivered a stinging forehand winner to seal the victory.

The 18-year-old was as surprised as anyone in the near-capacity crowd at the 8,000-seat venue. A wide-eyed Bencic put her hands to her face and squatted down as the crowd roared.

The two players smiled as they shook hands and Bencic cried tears of joy in her courtside chair as fireworks went off in the distance.

"This is why I worked for 15 years," she said. "I mean the practice is not always easy. It was hard, it was tough. Sometimes I didn't want to and sometimes you feel very motivated. And all this work paid off today I think and this feeling is indescribable."

The right-handed Williams played with a bandaged middle finger on her left hand. She called it a 'sprain,' suffered after she slipped and fell in her quarter-final win over Roberta Vinci a night earlier.

Williams appeared to have problems with the toss on her serve at times. But she didn't think the injury would keep her out of action as she continues to prepare for the upcoming U.S. Open.

"I just really didn't play up to par today — what a professional tennis player should play like," she said. "I played like an amateur, to be honest."

Williams had 16 aces to just one for Bencic, but the American double-faulted 12 times. She touched 200 km/h with her serve at times, but lost her rhythm midway through the second set.

At deuce while trailing 3-4, Williams served up a soft second serve that Bencic pounced on for a break point. She followed with a winner and Williams smashed her racket three times on the court in frustration.

Bencic, the unseeded world No. 20, needed another break to avoid a tiebreaker and earned a double-set point in the pivotal game. Williams double-faulted again as a giddy Bencic smiled at her good fortune.

"Today was not my day and hopefully I can rectify that going into next week and then going into hopefully the Open," Williams said. "Hopefully I can have a few more better days."

Williams often talked to herself between points and emphatically gestured in the direction of her coach. A poor start to the third set proved to be too much to overcome.

Bencic, who won her first career title this year at Eastbourne, became the youngest player to beat Williams in a completed match since a 17-year-old Maria Sharapova beat her at the 2004 WTA Finals.

Halep, meanwhile, will appear in the Rogers Cup final for the first time and is looking for her fourth crown of the year. The 23-year-old has won 11 WTA titles over her career.

The second-seeded Romanian needed 95 minutes to complete her semifinal victory on a hot, sunny afternoon. She took over a month off after a first-round loss at Wimbledon and feels energized since her return.

"I've had a great week," Halep said. "And you know, when you come to the tournaments without expectations, you are more relaxed and you have to play without pressure, play (like) during the practices and just trying to play your best every match.

"So I did this and I don't know how I'm in the finals, to be honest. But it's nice and I have just enjoyed (it)."

In doubles semifinal play, third-seeded American Bethanie Mattek-Sands and Lucie Safarova of the Czech Republic posted a 6-4, 7-6 (5) win over Kristina Mladenovic of France and Karolina Pliskova of the Czech Republic. Top-seeded Martina Hingis of Switzerland and Sania Mirza of India dropped a 6-3, 6-2 decision to fourth-seeded Caroline Garcia of France and Katarina Srebotnik of Slovenia in the late match.

Notes: Former WTA star Justine Henin — a two-time Rogers Cup champion — was inducted in the Rogers Cup Hall of Fame on Saturday evening. ... Bencic was named 2014 WTA Newcomer of the year after rising 179 positions in the world rankings to finish the season at No. 33.

———

Follow @GregoryStrongCP on Twitter.