NEW YORK — 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.