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.

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