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 either 23rd seed Donna Vekic of Croatia or Germany's Julia Goerges, the 26th seed.--