NEW YORK -- 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 on Monday.

The result also means that Osaka 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."

"Right now I have this feeling of sadness, but I also feel like I have learned so much during this tournament," Osaka said. "Of course I wanted to defend this tournament. So I hope that I can keep growing. I know that if I keep working hard, then of course I'll have better results."

A positive for Osaka was that she felt she improved her game over the summer. "I don't feel like I put so much weight on one single match," she explained. "As a whole I want to see that I played better matches through the summer. Because I feel like definitely the tournament that I played here has been the best one over the summer so far."

Bencic got off to the faster start, dominating with the early strike, depth and 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 in one hour and 26 minutes.

Bencic will in the quarterfinals play against Donna Vekic of Croatia who defeated Julia Goerges from Germany in another fourth round match.