NEW YORK, Sept. 2 -- Naomi Osaka of Japan ended her title defence at the US Open as the top seed lost to Belinda Bencic of Switzerland 7-5, 6-4 in the fourth round while Donna Vekic of Croatia and Elise Mertens from Belgium advanced into last eight with quality wins on Monday.
Osaka's fall also means that the Japanese superstar will lose the world no. 1 ranking after the tournament to Roland Garros champion Ashleigh Barty of Australia.
"Obviously I always adapt to my opponent, and that's just how I play," Bencic said. "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. I definitely try to do that against anyone, not only against her. Just taking the ball early and anticipating her. I think my game probably matches up well against hers."
Osaka agreed, saying: "She played pretty clean. I think she had a specific plan she wanted to execute."
Bencic got off to the faster start, dominating with the early strike, depth and with easy redirection on her backhand and breaking in the opening game for a 2-0 lead in the first set.
Osaka gathered herself, saving all four break points in the third game and overpowering Bencic with her forehand to break back for 2-2.
Bencic then scored another break in the 11th game and took the first set at 7-5.
Bencic continued her top form in the second set and took a decisive break in the fifth game before securing the victory in one hour and 26 minutes.
Coming up next for Bencic will be Vekic, the 23rd seed of the tournament. She saved a match point in the second set and made her first Slam quarterfinal by rallying past Julia Goerges from Germany 6-7(5), 7-5, 6-3.
Bencic owns a 2-1 career edge over Vekic from two 2017 wins but Vekic won the most recent meeting with her friend and frequent practice partner in the third round of this year's French Open.
"It's going to be a tough match for sure," Vekic said. "She's one of the best players this year."
Elsewhere, the 25th-seeded Mertens dropped just two games against American wildcard Kristie Ahn to reach the US Open quarterfinals for the first time.
During the 6-1, 6-1 demolition, Mertens fired 17 winners to Ahn's nine and kept her unforced errors in check -- 15 to her opponent's 25. The Belgian star stayed in command during the big points and saved all three break points she faced.
"I'm just trying to focus every time I step on the court, just trying to look point per point," Mertens said. "I was trying to play aggressive, trying to improve myself. That's probably the main key. Just try to focus throughout the whole match."
"I thought she played very smart. She had a game plan, and she executed it very well," Ahn said of the match. "I think it had to do with her kind of breaking my rhythm or not allowing me to have rhythm. I don't think she allowed me to see the same ball twice."
Mertens will take on Canadian player Bianca Andreescu in the quarterfinals. Andreescu defeated American qualifier Taylor Townsend 6-1, 4-6, 6-2.
"Andreescu has an all-around game. She's on fire when she's on the court. It's going to be really tough," Mertens said.