NEW YORK (AP) -- Naomi Osaka's achy left knee didn't let her serve without pain, so she didn't practice that key part of her game leading into the U.S. Open. The knee also prevented her from covering the court and preparing for shots the way she'd like.
Those weren't the only reasons that the No. 1-seeded Osaka's 10-match winning streak at the U.S. Open and title defense ended Monday in the fourth round. Belinda Bencic's clean, crisp strokes, struck with the ball still on the rise, contributed plenty to the outcome, too.
Osaka joined 2018 men's champion Novak Djokovic on the sideline before the quarterfinals, exiting with a 7-5, 6-4 loss to the 13th-seeded Bencic under a closed roof at Arthur Ashe Stadium on a rainy afternoon. Djokovic stopped playing in his fourth-rounder Sunday night because of a problematic left shoulder.
"I honestly didn't move well today. You know what I mean? I felt like I was always flat-footed. ... The knee was a little bit annoying in the movement aspect," Osaka said. "But I think that that's something I should have overcome."
As for her powerful serve, Osaka called it "inconsistent," saying she hadn't been working on it coming into the year's last Grand Slam tournament "because I can't really land on my leg that great."
Osaka has been wearing a black sleeve on the knee and was given a pain-killing pill by a trainer midway through the second set Monday.
By then, Bencic was up a set and a break, employing a quick-strike style of taking balls early and snapping them back, rushing Osaka and not leaving her not enough time to respond. It worked before: Bencic is now 3-0 against Osaka in 2019.
"I don't have the biggest power. Don't have the most winners or most aces. But I think I can really read the opponent's game well," said Bencic, who will face No. 23 Donna Vekic of Croatia in the quarterfinals. "I definitely try to do that against anyone, not only against her."
Bencic finished with far more winners, 29, than unforced errors, 12, and showed once again that she is a big-match player. She owns a tour-leading nine victories over top-10 opponents in 2019 and is 4-1 for her career against top-ranked players.
Bencic is 22, just a year older than Osaka, but her progress was slowed in recent years by injuries, including wrist surgery.
Back in 2014, when she was 17, Bencic became the youngest woman into the U.S. Open quarterfinals since 1997, when another Swiss woman, Martina Hingis, took the title.
Hingis' mother, Melanie Molitor, used to coach Bencic, and five-time major champion Hingis herself has served as a mentor. Bencic said she likes to emulate the way Hingis used to play, always thinking a move or two -- or more -- ahead.
"With Melanie, we didn't try to copy Martina's game. We tried to make my own game. And obviously, I know there are similarities, because that's the way Melanie teaches, but it was about making my own strengths and my own game style," Bencic said. "I play, of course, a little bit different than Martina. I think she was even more skilled and smarter on the court and playing more chess. I think I have a little bit less maybe talent and touch than her, but maybe a little bit more power."
In men's action, No. 6 Alexander Zverev was undone by 17 double-faults and bowed out to No. 20 Diego Schwartzman 3-6, 6-2, 6-4, 6-3.
"My first serve is still fine. My second serve needs to be worked on," Zverev said. "But I'll deal with it."
Schwartzman's quarterfinal foe will be No. 2 Rafael Nadal or No. 22 Marin Cilic, who played Monday night.
No. 24 Matteo Berrettini gave Italy its first U.S. Open men's quarterfinalist since 1977 and made it this far himself for the first time at any major with a 6-1, 6-4, 7-6 (6) victory over Andrey Rublev. Berrettini now plays No. 13 Gael Monfils, who overwhelmed Pablo Andujar 6-1, 6-1, 6-2.
Osaka made her breakthrough at Flushing Meadows a year ago, winning her first major championship by beating Serena Williams in a chaotic final that devolved after Williams got into an extended argument with the chair umpire.
Osaka followed that up with a second consecutive Grand Slam trophy at the Australian Open in January. That allowed her to become the first tennis player representing Japan to reach No. 1 in the rankings.
This loss means that Osaka will cede that top spot to No. 2 Ash Barty, who lost her fourth-round match Sunday.
"Right now, I have this feeling of sadness," said Osaka, who lost in the third round at the French Open and first round at Wimbledon, "but I also feel like I have learned so much during this tournament. Honestly, of course, I wanted to defend this tournament."
Another women's quarterfinal will pit No. 25 Elise Mertens of Belgium against No. 15 Bianca Andreescu of Canada or qualifier Taylor Townsend of the U.S. Mertens advanced by beating wild-card entry Kristie Ahn of the U.S. 6-1, 6-1. Ahn carried heavy tape jobs on her right arm and left leg.
Vekic, a 23-year-old from Croatia, reached her first Grand Slam quarterfinal by saving a match point and edging No. 26 Julia Goerges of Germany 6-7 (5), 7-5, 6-3.
"I don't even know how I won this match," Vekic said.
Well, here's how: Goerges served for the victory at 5-4 in the second set, coming within one point of ending things right there. Not only couldn't Goerges convert that match point, but she also double-faulted three times in the game and unraveled from there, wasting a 21-ace effort.
"It's not about that service game," Goerges said, perhaps trying to persuade herself.