Defending champ Osaka knocked out by Bencic

Top-ranked defending champion Naomi Osaka was ousted from the US Open on Monday by Belinda Bencic while Croatian Donna Vekic erased a match point to join the Swiss 13th seed in a quarter-final matchup.

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 atop the rankings next week and won't become the first repeat US Open winner since Serena Williams in 2014.

"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 Monday 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. "I'd like to reach higher rounds. That's definitely what I'll aim for in Australia."

Osaka's exit means there will be four different women's Grand Slam winners in a season for the third consecutive year, a first in the Open era.

Osaka took a pain-killer to ease nagging knee issues but nothing could help her overcome Bencic.

"I didn't move well. I was always flat-footed. She was aggressive," Osaka said. "The knee was a little bit annoying in the movement aspect but that was something I should have overcome."

Osaka struggled from the start on the same Arthur Ashe Stadium court where she consoled tearful US teen Coco Gauff just two days earlier.

Bencic hit a backhand down-the-line winner to break for a 6-5 lead then held on a service winner to capture the first set.

Osaka, who had 21 unforced errors, double-faulted away a second-set break to hand Vekic a 3-2 lead while the Swiss never faced a break point.

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

Belgian 25th seed Elise Mertens cruised past 141st-ranked US wildcard Kristie Ahn 6-1, 6-1 in 67 minutes to book a quarter-final berth against 19-year-old Canadian 15th seed Bianca Andreescu.

Andreescu, whom the Belgian has never faced, extended her career-best Slam performance by defeating 116th-ranked US qualifier Taylor Townsend 6-1, 4-6, 6-2.