Swiss 13th seed Belinda Bencic stunned top-ranked defending champion Naomi Osaka on Monday to reach a US Open quarterfinal against Donna Vekic, who rescued a match point to advance.

Bencic's 7-5, 6-4 triumph, her WTA-best sixth over a top-five foe this year, ensured 21-year-old Japanese star Osaka will fall from the world No.1 spot next week.

"The challenge cannot be bigger against Naomi," Bencic said. "I had to be at the top of my game and I'm really pleased how well I handled my nerves at the end."

Bencic, 22, matched her deepest career Grand Slam run from the 2014 US Open with her third victory of the year over Osaka, having also won at Indian Wells and Madrid.

"She played pretty clean. She had a specific plan she wanted to execute," Osaka said. "I guess just being aggressive."

Australia's Ashleigh Barty, the reigning French Open champion, will move atop the rankings next week after the loss by Osaka, who defends her Australian Open title in January.

"I can look at this and be very disappointed and mad, but I'm not mad about it," Osaka said. "Of course I'd like to reach higher rounds. That's definitely what I'll aim for in Australia."

Vekic, the 23rd seed from Croatia, saved a match point in the second set and made her first Slam quarterfinal by rallying past German 26th seed Julia Goerges 6-7 (5/7), 7-5, 6-3.

"I don't even know how I won this match," Vekic said. "She was serving for the match. She had match point. I just kept fighting and believing I could win. It feels pretty amazing."

Bencic owns a 2-1 career edge over Vekic from two 2014 wins but Vekic won the most recent meeting with her friend and frequent practice partner in this year's French Open third round.

"It's going to be a tough match for sure," Vekic said. "She's one of the best players this year."

Also moving into the last eight was Belgian 25th seed Elise Mertens, who cruised past 141st-ranked US wild card Kristie Ahn 6-1, 6-1 in 67 minutes.

"I just tried to play my best game," said Mertens, who has dropped only 16 games in four matches.

"I try to improve every time. I try to invest in my game. It's coming out this week and hopefully I can go much further than this."

Mertens, whose only deeper Slam run was into last year's Australian Open semifinals, will play for a berth in Thursday's semifinals against the winner of a later match between 116th-ranked US qualifier Taylor Townsend and 19-year-old Canadian 15th seed Bianca Andreescu.