DEFENDING champion Naomi Osaka has been knocked out of the US Open, losing 7-5 6-4 to Switzerland’s Belinda Bencic in the last 16 on Monday.

Osaka, 21, was broken late in the opening set and then again in the fifth game of the second at Flushing Meadows.

It means the Japanese player will lose her world number one ranking, with French Open champion Ashleigh Barty of Australia to return to the top spot.

For Bencic, 22, it is her second appearance in the quarter-finals.

She also made the last eight in 2014 and will now play Croatia’s Donna Vekic after the 23rd seed beat Germany’s 26th seed Julia Goerges 6-7 (5-7) 7-5 6-3 in a match that lasted two hours 42 minutes on Louis Armstrong Stadium.

“I was so excited to come on the court, the challenge cannot be bigger – Naomi Osaka is a great player and won the US Open last year,” said Bencic. “I had to be on top of my game and am really pleased with how I played.

“She has a lot of power, I was just trying to play it a little bit like chess, anticipate and make a tactic on the court.”

Osaka, who beat Serena Williams in last year’s final for her maiden Grand Slam title, started off badly as she lost her serve at the first opportunity and had to save two break points to avoid going 3-0 down.

She then fought back with a break before Bencic took the decisive break in the 11th game and then served out the set.

In the second set, an Osaka double fault gifted Bencic another break and the Swiss player took the final game of the match with a service game to love.

After winning the Australian Open in January for her second Grand Slam, Osaka became the world number one, a position she held until June before Barty took over.

Osaka returned to the top eight weeks later but Barty will become number one again, despite losing in the last 16 to China’s Qiang Wang on Sunday.