Bencic got off to a bright start on a rainy day in New York, breaking the world number one under the closed roof of Arthur Ashe Stadium, and her superior serving carried the day as she won 82% of her first serve points.
Momentum swung firmly in the 22-year-old Bencic’s direction in the second set when Osaka double-faulted to hand the 13th seed a 3-2 lead and the top-seeded Japanese, who appeared to struggle with a troublesome left knee, was unable to recover.
Bencic improved to 3-0 this year against the 21-year-old Osaka, who came through an emotional straight sets win over 15-year-old American Coco Gauff in the third round on Saturday.
Osaka’s defeat means Australia’s Ashleigh Barty, who lost in straight sets to China’s Wang Qiang in the fourth round on Sunday, will regain the world number one spot from the Japanese.
Osaka said that although she was disappointed with Monday’s result she still considered her trip to New York a success.
“Right now I have this feeling of sadness, but I also feel like I have learned so much during this tournament,” she told reporters.
“The steps that I have taken as a person have been much greater than I would imagine at this point.
“So I hope that I can keep growing. I know that if I keep working hard, then of course I’ll have better results.”
Big challenge
Bencic, who also reached the US Open quarter-finals in 2014 when she was just 17 but has struggled with injuries in recent years, said that taking Osaka’s powerful serve early and keeping her energy up was the key to winning the match.
“I was so excited to come on the court, the challenge could not be bigger so I had to be on top of my game,” she said in an on-court interview.
“I’m really pleased with how I played and how I managed my nerves in the end,” she added.
“I’m not the person with the most winners or aces, I had to play it a bit like chess with tactics on the court.”
Despite the lack of a big serve, Bencic played fluently, finishing with 29 winners to just 12 unforced errors and breaking Osaka three times.
Osaka’s loss comes the day after top seed Novak Djokovic fell to another Swiss, Stan Wawrinka.
Next up for Bencic is a meeting with Croatia’s 23rd seed Donna Vekic, who beat Germany’s Julia Goerges 6-7(5) 7-5 6-3 on Louis Armstrong Stadium court earlier on Monday.