NEW YORK • Naomi Osaka's achy left knee did not let her serve without pain, so she did not practise that part of her game leading into the US Open. The knee also prevented her from covering the court and preparing for shots the way she would like.
Those were not the only reasons that the top seed's 10-match winning streak at Flushing Meadows and title defence ended on 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, and Osaka exited with a 7-5, 6-4 loss to the 13th seed at Arthur Ashe Stadium.
"I honestly didn't move well today. You know what I mean? I felt like I was always flat-footed," the Japanese, 21, said. "The knee was a little bit annoying in the movement aspect. But that's something I should have overcome."
As for her powerful serve, Osaka called it "inconsistent", saying she had not been working on it coming into the year's last Grand Slam tournament "because I can't really land on my leg that great".
She has been wearing a black sleeve on the knee and was given a pain-killing pill by a trainer midway through the second set.
By then, Bencic was up a set and a break, employing a quick-strike style that has worked before. She is now 3-0 against Osaka this year.
"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 in the quarter-finals.
Belinda Bencic's perfect record against Naomi Osaka this year.
The Swiss 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 this year 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.
In 2014, when she was 17, she became the youngest women's US Open quarter-finalist 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.
Bencic said: "With Melanie, we didn't try to copy Martina's game. We tried to make my own game. I know there are similarities, but it was about making my own strengths and my own game style.
"I think I have a little bit less maybe talent and touch than her, but maybe a little bit more power."
In a summit tug of war, Osaka will cede the top spot back to No. 2 and French Open winner Ashleigh Barty, who lost her fourth-round match to China's Wang Qiang.
"Right now, I have this feeling of sadness," said Osaka, who lost in the French Open third round and Wimbledon first round.
"But I also feel like I have learnt so much during this tournament."
Serena Williams is the only player left who has won a Slam singles title. She has 23 Majors, one short of Margaret Court's record of 24. She will play Wang in the last eight this morning (Singapore time).
ASSOCIATED PRESS, NYTIMES